Luminous Landscape Forum
Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: victoraberdeen on December 04, 2003, 02:44:48 am
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Yes, hard to follow!
However DoF is not changed by the size of image sensor. I think the reference to larger f-stop is about the amount of light falling on the sensor?
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When comparing different format sizes, you must compare like physical apertures, not like f-numbers, when discussing DOF. Small format cameras do not offer "good DoF at those larger apertures" but they do offer "good DoF" at lower f-numbers since a given physical aperture represents a lower f-number on the smaller format.
Exactly Craig; thank you for the concise statement.
In practice, smaller format digital cameras so far have more DOF even wide open because their maximum apertures are smaller, despite often having somewhat lower (faster) aperture ratios. With film formats, this is usually not true: maximum apertures on lenses of similar FOV are rather similar all the way from "large large format" (8"x10") down to 35mm.
So the question is whether this trend can continue further. To match the aperture of f/2.8 in 35mm format requires about f/2 for APS formats, f/1.4 for 4/3" format, f/0.7 for 2/3" format. Given that some digicam zoom lenses already reach f/1.8 wide, or f/2.4 tele, getting a zoom down to f/1.4 at least seems quite realistic when you remember that the aperture and front element size is not getting any bigger. f/0.7 though ???
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Indeed, photographers moving down in film format size have very rarely used either of those strategies: instead, the primary change has always been seeking roughly the same composition (perspective, depth of field, and even degree of diffraction limitation) by moving to a shorter focal length, proportionately lower aperture ratio (same aperture diameter), and hence some combination of higher shutter speed and lower speed film (or lower sensor "ISO" sensitivity setting).
Yes, exactly!
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Ray,
perhaps the closest we will come to agreement is that, in your somewhat unfortunate situation of using a camera (D60) with a sensor of one size in conjunction with lenses originally designed for a larger sized format, you are often constrained to either use a lens of longer than ideal focal length, and hence forced to back off in order to fit the image, or to use a slow, wide angle lens. Blame lens limitations, not the format!
I really want to remove issues of current, quite likely temporary, limitations in lenses available for newer smaller formats from claims about inherent advantages or disadvantages of one format over another, so can I ask you some questions?
Imagine that you have access to any lens you desire for any format; any focal length, any maximum aperture, etc., with no size, weight or cost worries; and you wish to make a particular photograph with one of several cameras of different format sizes. That is, think only of artistic considerations for a moment.
a) would you desire to use the same subject distance and perspective in each case, or to change both when changing formats?
would you desire the same depth of field in each format, or to change this when changing formats?
Moving back towards reality by bringing size and weight limits into the picture only adds more flexibility for smaller formats, which fairly consistently achieve the same maximum aperture diameter in the shorter needed focal length with a somewhat smaller (and cheaper?) lens.
That brings me to your second point about long, fast lenses for wildlife photography and such. I am not sure how relevant it is to the topic at hand, but you are quite right in pointing out that it is harder to do that sort of photography in larger formats, which relates to the relative lack of such lenses for MF and LF. This is because of one of the true inherent differences between formats: larger formats generally require longer exposure times and longer lenses, and both of these make them less suited to extreme telephoto work.
But this is straying very far from the original DoF discussion.
About my shallow DOF portait example, it is more thanteh perdominance of 35mm. After all, for professional portraits, MF is a rather big player. And yet work out what would be required to match the shallow DOF of a 35mm format 85/1.4 with its 61mm maximum aperture. The only medium format lenses that can match that aperture are 180/2.8 and longer, so the narrower FOV requires backing off and gives more DoF; nothing in LF can do better either. The same happens with a 135/2, and for more or less any fast 35mm lens: there is no lens in existence for larger formats that will give you lower minimum DoF for the same subject framing.
So both basic optical theory and and the practicalities of lenses available for established formats contradict the idea that larger formats have increased shallow DoF potential.
P. S. Probably a 300mm lens for 35mm format could be used on a MF or even 4x5 camera with no vignetting if you avoid the lens hood; telephoto lenses tend to have a natural angular field of view similar to a normal lens (about 50 degrees) and hence vastly excessive image circles, which are then cropped in the camera. (This is the refutation of the common criticism that smaller formats involve more cropping: telephoto lenses for any format with a given telephoto field of view are usually cropping about the same fraction out of a roughly 50 degree FOV.)
One way to see this is to look at specifications for large format lenses, which often give their angular field of view to let users know which formats and movements they can handle. Wide and super wide lenses offer large angles, but as the focal lengths move up well into telephoto territory, the angular FOV rarely or never drops below 50.
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Ray,
why do I get the feeling that you do not want to answer the question that you know I am asking, and keep instead reinterpreting it?
JBL,
Probably because I'm a practical person who is after the best results with the equipment I have and because in practice I simply would not merely transpose f stop and shutter speed with a different format camera in the manner you are suggesting.
Lack of sufficient DoF is a common constraint in photography as you well know. It would be great if one could use f32 with one's 35mm camera and get similar resolution to f8 as well as using the same shutter speed.
What you are doing is setting up a hypothetical situation and asking how I would respond without regard to restraints when the artistic concept itself, right from the beginning, would have restraints built in.
Let me give you a concrete example from recent experience. Many art galleries, museums, historic homes etc have rules about photography. Sometimes no photography at all is permitted. Sometimes photography is permitted but no flash or tripod allowed.
In such circumstances one struggles with the competing constraints of shutter speed and DoF. One usually ends up with a compromise, or no shots at all; perhaps f5.6 at 1/30th is the best one can do.
Enter a nice gentleman like yourself who hands me a Leica Digicam; do I now use F2 at 1/250th? Almost certainly not. I might well stick with the same f stop and shutter speed, thus lifting those constraints, achieving greater DoF and a more appropriate shutter speed for a hand held shot with the shorter lens.
Why are you trying to get me to give you an 'unreal' answer?
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DoF is dependent on sensor size
No Ray you didn't make that mistake
However, there are other differences between the lenses produced for different formats that have more serious effects. There are two distances that matter here, subject to nodal point and nodal point to image plane. It is the latter that changes with large format cameras and is the reason you are all failing to understand this topic.
With DSLR's it is only a crop the same crop on a full frame image will be exactly the same.
Sharpness is a compromise...
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Ray, No assumption, the focal length needs to be the same. However to retain the subject retain the same percentage size in the image, you would have to assume the leica has the right focal length!
And rude, never
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... 2/3rds a silly term really - for them that still use inches
2/3", 4/3" are stage anachronisms, and I find 1/1.8" even weirder (why not 5/9?), but here is the trivia question for the day:
To what important photographic dimension does "35mm" refer to?
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Wierder to me is that the focal length scale on the lens is in "35mm equivalents" rather than real focal lengths. So far those equivalents have been hidden in electronic dispays. (Apart from purism, I disagree with the standard choice of conversion factor, since it assumes comparing to 35mm format by cropping to 35mm frame shape and so using full frame width but discarding some height; I am more likely to use the full frame height, possibly discarding some width from either shape when cropping for shapes like 10x8" or US letter.)
On the other hand (or other side), I like the idea of having the LCD as big as it can possibly be, filling the entire height of the back panel.
Nice to be talking abut the original subject of this thread at last, on page 8!
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DP Review is reporting the announcement of the new Leica Digilux 2.
Digilux 2 (http://www.dpreview.com/news/0312/03120101leicadigilux2.asp)
Hope it's fast!
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Yes, hard to follow!
However DoF is not changed by the size of image sensor. I think the reference to larger f-stop is about the amount of light falling on the sensor?
However DoF is not changed by the size of image sensor.
I've got a sense of deja vu here. If the only change is the size of the sensor, then it's true that DoF does not change, just as merely cropping an image does not change DoF and just as fitting a 35mm back to an LF camera does not change DoF.
However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
I think the reference to larger f-stop is about the amount of light falling on the sensor?
Increasing the aperture (reducing the f stop number) always increases the amount of light passing through a lens. However, it's only the small format camera that can offer a good DoF at those larger apertures. The advantage of this is, for the same scene and angle of view, one can use, say F 2 at ISO 100 and 1/125th sec instead of F 8 at ISO 400 and 1/60th sec.
With my D60, the noise at ISO 400 is not significantly less than the noise one gets underexposing the shot by using ISO 100 and the same shutter speed.
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However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
That's because when you adjusted the focal length, you changed the physical aperture to keep the f-number constant. DOF didn't increase because of the smaller sensor, it increased because you closed the physical aperture!
Um yeah, true. What is your point?
I don't get why people argue this over and over and over again. Skip all the math, skip all the theorizing. Try this:
Take a small sensor camera. Make it a P&S, a 35mm, what ever. Select a 3D scene. Select an exposure; lets say 1/250 @ F8 @ ISO 100. Take picture.
Now take a 4x5. Insert ISO 100 Film (or a digital back if you are quite well off). Select focal length for the same composition from the same position. Select the same exposure. Do not use movements. Take picture.
Print both at the same size; lets say 8x10. Visuallly inspect. Do they have the same DoF? I can assure you it is not even in the same ballpark. The DoF in the 4x5 will be very very very thin. The P&S DoF will be very good.
Now argue all you want on why, but it still holds in the real world: DoF EFFECTIVELY depends on the sensor size. You can blame it on focal length if you want, you can point at this variable or that changed, argue to you are blue in the face, but you will still have more DoF for your desired composition on the small sensor camera.
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The DOF advantage myth of small digital formats comes from two sources. First, people are accustomed to shoot something at f/8 and 1/60 at ISO 100 (for example) regardless of format. They observe (rightly) that the smaller format has better DOF but fail to realize that f/8 is NOT fixing the physical aperture. The second source is that these smaller sensors don't have lenses that can open up like faster 35mm lenses do, so they never see restricted DOF from these cameras. Their conclusion is that smaller formats have better DOF accross the board when that is not true at all.
Larger format film shooters also see DOF as limited but for a different reason. Larger formats are optimized for larger print sizes so the CoC and diffraction tolerance don't scale with the imager size. MF is not optimized for high magnifications and 8x10 prints. If it were it would exceed the performance of 35mm in DOF at the cost of resolution.
I don't think you give other people enough credit. Some people -- like me -- have a different basis for evaluation. I am well aware of the "truth", but prefer to look at things more realistically. So I take issue with your argument not because it is wrong (it isn't), but because it is a distraction from the real outcome. Those people don't necessarily fail to realize that the physical aperture has changed, but perhaps they like shooting at 1/60. What the technical version of the argument fails to account for is the real world. Indeed -- in the real world -- it can be very difficult to limit DoF sufficiently with small sensor cameras. Incidently, it is quite easy to achieve very limited DoF with large format, but rarely desirable for two reasons: 1) LF shooters are extremely shallow DoF types of people for the most part and 2) LF lenses suck wide open. This is because they are simple, light and there is little motivation to change this because of (1)
35mm is the only format where people happily strap on very large heavy lenses to with the resultant shallow DoF. For the most part, equivilants to these lenses are not available in other formats. All participants here know this, but choose to argue the technical aspects rather then the outcome: In the real world, a small sensor camera will have difficultly restricting DoF. Maybe they will release a F/0.7 lens on a small-format camera in the future with ISO 16 so you can both limit DoF and have a slower shutter speed, but I have my doubts.
Incidently, Macro lenses are certainly available for both MF and LF. However, the extremely small F/stops required to get sufficient DoF make for very long exposures (or massive strobes) and make them more then a little inconvient to use. Outside of the studio, even the slightest breeze will destroy your shot. Just one part of the reason why you don't see them used much.
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I noticed you didn't refute anything I said.
Yes I did. You tried to say that F stop and DoF considerations do not apply across formats. When I challenged you on this point, you then wrote that it only applied to exposures and it was nuts to pretend that a constant DoF across formats was relevant because people do not take photos of bits and pieces but whole compositions (or words to that effect).
What is nuts or irrelevant, or what constitutes a bit or a piece or a whole composition, is a matter of opinion; a matter of taste and a matter of esthetics. I'm trying to cut through your obfuscation with some facts.
That a 300mm lens at a given f stop will have the same size aperture and the same DoF (from the same shooting position)[/I]irrespective of the format the lens was designed for is surely of some academic interest to those who didn't already know it, and is surely of more than academic interest to owners of APS format DSLRs who didn't already know it.
The resultant improvement in DOF in this test is not due to the change in sensor size,
You seem to be looking for direct causes. Are you trying to say that only direct causes are valid? A lot of things that happen are a consequence of a consequence of a consequence etc. Exponents of Chaos Theory will have us believe that a butterfly flapping its wings in South America might just tip the balance and cause a storm in Australia (I wish it would. It's still pretty dry over here.) This is an extreme example and I don't take it seriously. That reducing sensor size might (and certainly will if you keep the f stop and FoV the same) result in an increase in DoF has a much more direct causality.
By the way, it's difficult to hide a phrase like "same f stop" in such a short sentence. I don't know how I managed that.
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Ray,
why do I get the feeling that you do not want to answer the question that you know I am asking, and keep instead reinterpreting it? Surely I have made it clear that I am not talking about constraints of lens availability (like tilt or no, better quality for one format than the other), or issues of frame shape (it is possible to crop, even though I know you prefer not to!), and I am certainly not talking about changing your mind about your artistic intent; I am talking about how you would go about achieving the SAME artistic intent, including framing after cropping if necessary, with cameras giving a different image size.
And I think that buried in your reply, you have agreed that, at least with the added assumption of same aspect ratio, you would indeed pursue a given intent by maintaining about the same subject distance and aperture diameter, and so different f-stop.
Since my original objection was to the accusations against smaller formats of constraining one to less shallow DOF options by arguing on the often tacit assumption that aperture ratio would be, should be, or usually is keep the same with different formats, I am now satisfied with such agreement as we have achieved.
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DoF is dependent on sensor size
No Ray you didn't make that mistake
Victor,
Well. thank you for that small mercy. I think perhaps it is really you who is responsible for this constant flow of diarrhea from Craig, aided and abetted by BJL
I'll refer you to my first response to you on page 1 of this thread. You seem to be suffering from a memory lapse.
I've got a sense of deja vu here. If the only change is the size of the sensor, then it's true that DoF does not change, just as merely cropping an image does not change DoF and just as fitting a 35mm back to an LF camera does not change DoF.
However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
Is there anything in the above two paragraphs that is (a) incorrect, or ( indicates that I don't understand the difference between cropping an image and changing the focal length of the lens. I think not.
What I really don't understand about this topic is how anyone can misinterpret such plain, factual and clearly expressed statements. That's a real mystery. :D
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You can get the photo with any camera, Leica, Rollieflex, MPP exposing a 30th at f5.6 and get a publishable photograph, if the focal length of the lens is the same you’ll even have approximately the same depth of field. The light making the image is the same regardless of image size. Ergonomics is another story!
There is an implicit assumption in this example that the maximum focal length of the Leica digicam is less than the focal length of whatever lens was being used at F5.6 on the 35mm camera.
I don't wish to be rude, but I sometimes get the impression that you are failing to see the woods for the trees. But then you probably feel the same way about me :) .
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What is measured is simple enough, but never used becaue we all use the Length x depth. But worse it is in inches why not mm!
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Some of my Canon lenses have a mask inside the lens to help reduce flare. Looks similar to this.
Gordon
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(To be read with an aristocratic demeanor...)
It is a Leica!
It has proper controls, not those absurd menus and buttons!
It has an EVF??????
(Let the rationalization tap dance begin.... )
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For a smaller sensor the actual focal length is shorter than than it would be for a larger sensor.
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Agreed, with one crucial qualification: shifting to a larger format only fully gives this advantage when it allows one to gather more light, through some combination of larger aperture diameters (less DoF etc.) and longer exposure times that would not possible in the smaller format because it would lead to overexposure.
BJL,
But isn't this exactly what happens? The larger the format, the larger the f stop number required, the longer the exposure required and the greater the need for a tripod. It's all very cumbersome and only for the truly dedicated.
Each format has its advantages and disadvantages but one should not forget that large format cameras usually have a tilt mechanism which allows the use of larger apertures whilst maintaining a reasonable DoF. I believe f 22 is the aperture required for stunningly sharp and detailed landscapes from a 10x8" camera.
Of course, one could argue that tilt mechanisms are also available for some of the smaller formats. I own a 35mm tilt&shift lens myself, but I don't see any resolution advantage to the larger apertures. 35mm lenses in general tend to have their best performance at either f8 or one or two stops down from maximum aperture. I suppose it's possible to design, say an f2.8 lens that has its best performance at full aperture, but such performance is likely to be only marginally better than at f 8.
What would be required for a small digicam to compete with a full frame 35mm, irrespective of the number of pixels on the sensor, is an F 2.8 lens that has significantly better resolution at F 2.8 than the larger format 35mm lens has at f8.
Is this possible? Well, it might be some day. But aren't we talking Science Fiction? :D
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Ray,
as far as I can tell, everything you say is true about the optical relationships between focal length, image size formed in the camera (and so indirectly, camera format), aperture diameter, aperture ratio, and subject distance. However, I seriously doubt the relevance of your comparisons in most situations. I agree that they do apply sometimes, such as for the unfortunate photographer who has an "APS" format DSLR, wants to take wide angle images, but is still making do with a collection of 35mm format lenses!
The options you describe involve the photographer who is using a smaller format either (a) moving back, in order to use the same lens instead of a shorter focal length lens that would often be smaller and lighter, or ( using a smaller aperture ratio, perhaps in order to use the same shutter speed instead of a using a faster one. Neither seems like the first or second choice in most situations!
Indeed, photographers moving down in film format size have very rarely used either of those strategies: instead, the primary change has always been seeking roughly the same composition (perspective, depth of field, and even degree of diffraction limitation) by moving to a shorter focal length, proportionately lower aperture ratio (same aperture diameter), and hence some combination of higher shutter speed and lower speed film (or lower sensor "ISO" sensitivity setting).
Why not compare on the basis of what photographers have actually been observed to do in most cases over the years?
A final note: the most extremely shallow DOF I have seen in film photography usually comes from the smallest mainstream film format, in things like portraits done with ultra-fast 35mm format lenses.
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For you folk who are getting excited about DoF, take a look at the DOF notes on Bob Wheeler's site, it is a very good explanation. notes on technical aspects of photography (http://www.bobwheeler.com/photo/Documents/documents.html)
Enjoy
Victor
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Sorry Ray your writing is just not clear. Please don’t expect me to take the rap for the commentary of others! This is a simple subject explained very well in Rob’s article.
Now the 'angle of view' is a good example of poor vocabulary, do you mean the perpendicular angle of the image plane to the subject or how many degrees the lens sees. The first one is correct, the number of degrees a lens captures to an image plane is the field of view.
Funny thing, to understand this subject you need drawings the like of Rob’s!
Now Ray save us folk from your rant and crawl back in to your f32!
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a) If, however you want the subject to have the same percentage size in the image, you’ll need to use different lenses and for the same depth of field range a different aperture.
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it is the wrong reason to choose your format.
Point a) is exactly what I am banging on about: larger formats will require larger aperture ratios to get the same depth of field with teh same framing, tending to force the use of longer exposure times when the opportunities for increasing ISO run out, which is less convenient for many moving subjects and some hand-holding situations
Point is mysterious; I would think that in many photographic situations, the ability to operate at higher shutter speeds is often a very good reason to use smaller formats, for greater hand-holdability to start with. In the film world, action photography (be it sports, wildlife or available light people photography) highly favours 35mm over larger formats for very much these sort of reasons, and lots of sport/action photographers are now enamoured of the advantages of the even smaller APS DSLR formats over 35mm DSLR's for what I believe are related reasons.
Anyway, I was not arguing that people should choose one format or another; I simply made a comment about what people should expect, and not expect, of the new smaller digital formats, in comparison to the more traditional, familiar larger formats.
That, and debunking the idea that very different format sizes should be compared on the assumption of using roughly the same f-stop when trying to get a similar image, which you iterate again in your opening paragraph suggestion about f/5.6 working well regardless of format.
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Ray, No assumption, the focal length needs to be the same.
The Leica Digicam has a 7mm-22.5mm zoom. The 2/3rds format (a silly term really) has a sensor diagonal of 11mm, equivalent to a 35mm format lens of 44mm. The equivalent f stop in the Leica for approx. same DoF as 35mm at f 5.6 (all else equal) would be F1.4, so even if I wanted to maintain the same DoF, I wouldn't be able to. Maximum aperture on the Leica is F2.
You might well ask, so what! And it's true that the resolution limitations of the really small format camera could make the camera useless for one's purposes. A pinhole camera has phenomenal DoF and you maight say, so what!, everything's slightly blurry, and you'd be right. It definitely wouldn't be suitable for magazine coverage.
But this is a different issue, isn't it? We're having a theoretical discussion here, aren't we? The subject is not, "How to make interesting photos by judicious use of DoF", or "How to make a million bucks by selling photos".
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I may be mistaken but I believe the chip designations come from the silicone chip world. They have their own way of naming and describing things. The fact that they have little relevance to photographers is to be expected. They still talk about furlongs at horse race tracks.
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Philibileboiles a word... :angry: :p :O :D :cool:
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It also has small 3.4 micron pixels and uses SD cards.
I had to laugh at the Leica PR writer that penned "Extra large picture storage card with 64 MB ".
Right. It'll hold 6 pictures in RAW mode.
All of that aside, it does look very appealing, and if they get all of the pieces to come together properly it could well be an attractive digicam.
A digital M series Leica it's not though, regardless of the cosmetics.
Michael
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BJL,
I see you haven't lost your enthusiasm for the smaller format. :D
It's a bit of an effort to follow your reasoning, but it seems to me you are essentially saying the smaller format has the advantage of smaller f stop numbers and faster shutter speeds for an equivalent DoF. I think this principle applies across all formats from a 16x20" field camera to the most miniature digicam.
However, there is an unavoidable trade-off. The smaller the format, the smaller the maximum print size in relation to a given standard of acceptable sharpness and the narrower the range of DoF settings (for acceptable sharpness). I see no way around this. Whilst advances in technology will no doubt eke out greater performance (lower noise and higher resolution) from the smaller digicam, such advances should also be able to be applied to the larger formats, thus maintaining those advantages of the larger format.
We should not lose sight of the fact that these small format digicams like the new Leica and the Canon G4 etc bear the same relationship to full frame 35mm as 35mm does to LF 4"x5".
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However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
That's because when you adjusted the focal length, you changed the physical aperture to keep the f-number constant. DOF didn't increase because of the smaller sensor, it increased because you closed the physical aperture!
Increasing the aperture (reducing the f stop number) always increases the amount of light passing through a lens. However, it's only the small format camera that can offer a good DoF at those larger apertures. The advantage of this is, for the same scene and angle of view, one can use, say F 2 at ISO 100 and 1/125th sec instead of F 8 at ISO 400 and 1/60th sec.
This is an example of the typical confusion between aperture and f-number. When comparing different format sizes, you must compare like physical apertures, not like f-numbers, when discussing DOF. Small format cameras do not offer "good DoF at those larger apertures" but they do offer "good DoF" at lower f-numbers since a given physical aperture represents a lower f-number on the smaller format.
People get away with linking aperture and f-number because they are used to dealing within the same format. When comparing different formats, you must be more careful. The DOF myth is due to lack of rigor.
DOF is a function of physical aperture and magnifcation. Ignoring lens performance, smaller formats offer slightly inferior DOF at equivalent, high magnifications. Exactly the opposite of what is commonly believed.
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pmkierst,
Depth of field choice is a significant part of desired composition, and the choice will tend to be the same regardless of format used; a particular numerical value of the f-stop used is not an inherent compositional choice. So it seems an outright contradiction to say that "you will still have more DoF for your desired composition on the small sensor camera."
It is rather obvious that people do NOT use the same aperture ratio when working in different formats, and lenses for different formats do not even offer the same choices of aperture ratio. Many lenses for 4x5 do not even get as wide open as f/8, whereas their counterparts for 35mm and smaller formats often go down below f/2. Indeed, larger format lenses do not offer such low f-stops as 35mm lenses because they would be huge, heavy and give ridiculously little DOF.
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You need to understand that holding the f-number constant across formats is meaningless and bogus. Once you figure that out, you'll realize the DOF claim is wrong.
Dear me! I don't quite know what to say, Craig! :D What sort of nonsense are you trying to perpetrate! ???
The F stop is one of the great concepts of Photography precisely because it is[/i] meaningful across formats. A 300mm lens at f8 will always require the same shutter speed in the same lighting conditions whether that lens is attached to a 35mm camera, a tiny digicam or a 12"x16" field camera, and it will always produce the same DoF from the same shooting position.
I've already mentioned in my response to Victor that 'only' changing the sensor size does not affect DoF. But you must realize that nobody designs and manufactures camera lenses without regard to the sensor size, film size, ie. format of camera the lens will be used with. However, it's true that lenses designed for a larger format can be and sometimes are used for a smaller format, the obvious example being the recent crop of DSLRs such as the D60, 10D and 300D.
So let's look at what happens when we take identical shots from the same position with a 50mm lens on a 1Ds and 10D. If the lens is the same and the F stop is the same and the distance to the central subject is the same . . . it follows that the exposure will be the same and the DoF will be the same. That's the beauty of the F stop system.
But something will be different, won't it? The 10D shot will be severely cropped. In oder to get the same angle of view with the 10D I'll have to step back a few paces. Consequently, the DoF will now be greater on the 10D shot because I've increased the distance between camera and subject. Alternatively, I could just whack on a 28mm lens to get the same field of view from the same position. That would also have the effect of increasing DoF for the 10D shot (at the same aperture).
Now you could argue that the increased DoF that the smaller format 10D seems to exhibit has nothing to do with its smaller sensor size and that the real reason for the increased DoF is the fact that you either (a) had to step back to get a wider FoV or ( had to use a wider angle lens.
However, there are reasons behind reasons. If the 10D did not have a smaller size sensor than the 1Ds, there would be no reason to step back or to fit a wider angle lens. In this example (which extends across all formats), the size of the sensor could be viewed as the more 'primary' cause of the increased DoF.
Even from a purely mathematical point of view, there is a connection between sensor size, F stop and aperture if we're talking about standards lenses for the different formats. F stop is a relationship between the focal length of a lens and the physical dimensions of the aperture (F=FL/D). The focal length of a standard lens (as opposed to a telephoto or wide angle lens) is given by the diameter of the image circle the lens throws on the sensor. The size of the sensor or format bears a very direct relationship to the image circle. All 35mm lenses for example are designed to have an image circle a bit larger than the diagonal of the 36x24mm frame (about 43mm). The diagonal of an 8"x10" plate for a field camera is about 12.8"= 320mm. It's no accident that the standard lens for an 8x10 field camera is just that . . . 320mm.
Get my point? :)
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OK, "F-Stop" numbers are dependent on focal length and physical aperture (and it is a non-obvious equation depending on exactly where the actual physical device is located). So saying F/8 for one lens and F/8 for another (different focal length) lens already accounts for the difference.
No it does not. Physical apertures are normalized by focal length for exposure purposes, not DOF. Two lenses of different focal lengths at f/8 have different apertures, not the same! The do, however, works the same way from an exposure point of view.
This fact, which few seem to understand, is why smaller sensors are preceived to have better DOF when they do not.
Your MF comments simply reflect the fact that MF is not optimized for high magnifications. It is not a testament to MF's inherent inability to deliver good DOF. If the lenses were made for it, things would be different. That's why I used FF 35mm for the macro discussion. It doesn't help to talk theoretically when you can test.
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Ray, if you're trying to be helpful by accusing me of spreading nonsense and posting to show off my knowledge, it sure doesn't make you look good. I noticed you didn't refute anything I said. Everything I said was correct and you apparently know that already.
This statement:
However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
is correct and I never questioned it or even commented on it. Your quotes that I did comment on were specifically provided.
In your example, the smaller format achieves better DOF by reducing magnification and closing the physical aperture. The second half of that is cleverly hidden within the "same f-stop" setting since you reduced the focal length. The resultant improvement in DOF in this test is not due to the change in sensor size, but you know that already since I'm not telling you anything you don't already know.
The reason this is not proof of superior DOF is simple. The accepted CoC translates to a smaller radius in the imager and results in a smaller acceptable level of diffraction. The maximum (smallest) acceptable aperture is therefore reduced. Nowhere is this more dramatic than in the P&S's where maximum aperture is often f/8 or less! This reduction in best aperture precisely offsets the reduction in required magnification. The whole f-stop issue is nonsense. When you talk about exposure you use f-stops. When you talk about DOF you don't. You cannot claim a DOF improvement due to focal length reduction when it is, in fact, due to an aperture change!
Ray, you obviously understand the difference between aperture and f-stop (which many apparently do not). I also believe that you understand that you understand the exposure issues surrounding larger versus smaller sensors. Let's take your 10D plus 1Ds example again only pretend there isn't any such thing as f-stop. We'll shoot a subject with the 1Ds+50mm using some midrange aperture and a 1/125 shutter. We then should the same subject with the 10D from the same location using a mythical 31mm lens to provide equivalent FOV. We use the same aperture but with a faster shutter to compensate for the inherent improvement in sensitivity that results from the gathered light being spead over a smaller area. Which one will have greater DOF? The 10D will by virtue of the smaller magnification only. If we maximize DOF by using the smallest acceptable aperture, the DOF of the two will be about the same until high magnifcations where the 1Ds will pull ahead. The last part of that is non-obvious but true nonetheless.
The DOF advantage myth of small digital formats comes from two sources. First, people are accustomed to shoot something at f/8 and 1/60 at ISO 100 (for example) regardless of format. They observe (rightly) that the smaller format has better DOF but fail to realize that f/8 is NOT fixing the physical aperture. The second source is that these smaller sensors don't have lenses that can open up like faster 35mm lenses do, so they never see restricted DOF from these cameras. Their conclusion is that smaller formats have better DOF accross the board when that is not true at all.
Larger format film shooters also see DOF as limited but for a different reason. Larger formats are optimized for larger print sizes so the CoC and diffraction tolerance don't scale with the imager size. MF is not optimized for high magnifications and 8x10 prints. If it were it would exceed the performance of 35mm in DOF at the cost of resolution.
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as far as I can tell, everything you say is true about the optical relationships between focal length, image size formed in the camera (and so indirectly, camera format), aperture diameter, aperture ratio, and subject distance.
Good! I'm glad we've cleared that up :D
As to whether or not some of my statements are relevant is something one could argue about forever. Relevant to what? Those of us who are creative are not restricted to fixed patterns of behaviour. We like to experiment a bit and try something different now and again. Whilst I personally would not go to the trouble of attempting to use a 300mm lens, designed for the 35mm format for example, on an MF body, I would not criticise someone for trying it. The circular image with a gradual fade-off at the perimeter could be quite interesting.
The options you describe involve the photographer who is using a smaller format either (a) moving back, in order to use the same lens instead of a shorter focal length lens that would often be smaller and lighter, or ( using a smaller aperture ratio, perhaps in order to use the same shutter speed instead of a using a faster one. Neither seems like the first or second choice in most situations!
That might be your opinion, but for me option (a) is often the first choice. When I do indoor shots with my D60, I often use my el cheapo Canon 50mm F1.8, partly because it's my sharpest lens; partly because I can stand back a few paces and get greater DoF at whatever aperture I choose and partly because, on a D60 the lens become effectively an 80mm lens which is more ideal for portraiture.
instead, the primary change has always been seeking roughly the same composition (perspective, depth of field, and even degree of diffraction limitation) by moving to a shorter focal length, proportionately lower aperture ratio (same aperture diameter)
Not entirely true. There are a lot of wild life photographers who actually use longer focal length lenses than the MF and LF photographers. Are there any 300-800 zooms available for 10x8 cameras? ???
the most extremely shallow DOF I have seen in film photography usually comes from the smallest mainstream film format, in things like portraits done with ultra-fast 35mm format lenses.
All[/i] photos are usually done with 35mm lenses, fast or slow because it's the most ubiquitous format we have. Although the paradigm is now changing, the number of extant 35mm photos must be staggering.
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Imagine that you have access to any lens you desire for any format; any focal length, any maximum aperture, etc., with no size, weight or cost worries; and you wish to make a particular photograph with one of several cameras of different format sizes. That is, think only of artistic a) would you desire to use the same subject distance and perspective in each case, or to change both when changing formats?
would you desire the same depth of field in each format, or to change this when changing formats?considerations for a moment.
It would depend on the subject, the circumstances and how much time I have available. With no time constraints and a large choice of lenses and different format cameras available, ideally I would like to try different lenses with different effective focal lengths, with different aperture settings and different effective DoFs, on different formats, creating different perspectives of the same subject from different angles and creating images of differing resolution for different degrees of print enlargement. (Provided the subject was interesting, of course.)
In circumstances where the subject would not lend itself to such experimentation, I would choose the format, the lens, the f stop and shutter speed I thought most appropriate and would try to balance the various trade-offs in making those choices. For example, if I were shooting in a dimly lit nightclub (or even a not so dimly lit place) and wanted a reasonable DoF and also wanted to remain inconspicuous, I might choose the smallest format camera available. If I were shooting distant wildlife I would almost certainly choose 35mm and the choice of aperture would then depend on such factors as whether or not the lens had Image Stabilisation and at which aperture the lens was sharpest, and whether or not using a tripod was practicable.
If I were shooting a panorama I would choose the largest format camera in my kit, but not if it was windy because it would be difficult to get a sharp shot. I would then make do with MF.
So you see, there are so many variables, a simple answer is not possible.
But I do get the impression you are rather envious of owners of the APS size DSLRs who have this huge choice of 35mm lenses that are effectively upgraded by the cropping factor. :D
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Ray,
you seem to have completely misunderstood my question, which in particular was NOT about deciding which format to use for a certain situation. .
BJL,
Perhaps I have misunderstood your question, but I suspect it is you who has not understood the implications of your question. Read your own question again.
.... access to any lens for any format .... think only of artistic considerations.
I think perhaps you are trying to elicit from me the straightforward answer along the lines .... if I had carefully composed my picture using a 35mm camera and had chosen 1/250 sec at f8, and then changed my mind and decided to use (or had been asked to use) a 6x9cm MF camera for the same scene, having already done the work from an artistic point of view I would then change the f stop to f16 (or maybe F13.5) and the shutter speed to 1/60th, having selected a lens of equivalent focal length to keep the FoV the same.
Maybe I would. I would certainly have good logical reasons to do this because, as everyone participating in this thread must surely know (and I know that you know that I know) f stop has to be increased for equivalent DoF as one moves up to a larger format.
However, there are lots of reasons why I might not abide by this simple transformation; some of them artistic and some purely logical. Get into the real world, BJL! :D
For a start, if I can change my mind about which format of camera to use for a particular scene, it's quite probable I would change my mind about some elements of the composition once I have a completely different camera in hand. It might just be due to the effect of squinting with one eye as opposed to looking at an 'upside down' or reversed image with two eyes.
It might be because I know that a particular lens does not have the required performance at a particular aperture, say not sharp in the corners at an equivalent aperture when the corners might in fact be important in that particular composition.
It might be because the different format is not just larger or smaller but of different proportions.
It might be because one camera has a tilt mechanism and the other doesn't.
In general, different formats with different lenses will have different strengths and weaknesses. It would be foolish to ignore these and just perform a simple transpose of F stop and shutter speed.
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It is then that you can freely choose to duplicate your photographic choices in another format. Once you do that, you will see that DOF is not dependent on sensor size.
It's actually quite difficult to accurately duplicate photographic results in another format, especially when moving down in format. There's usually a consequent loss in resolution and tonality.
Show me where I've written that DoF is dependent on sensor size and I'll apologise for getting it wrong. :D
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I can't believe this: five (5) pages of supertech mumbo-jumbo hyperbole about DOF without a single reference to the formula or to the almighty critical role of the "circle of confusion"! The latter is essentially the size of the circle created on the focal plane (film or sensor) by its intersection with the cone of light created behind the lens, by the lens, from a point of light in front of the lens. From a subject point that is precisely in focus, that cone of light converges exactly on the focal plane. From subject points that are out of focus, those cones of light converge in front of or behind the focal plane. The farther a point is out of focus, the farther the point of convergence from the focal plane and the larger the circle projected upon the focal plane. Depth of field is now the the distance between the nearest and farthest points, at a given aperture and lens extension that project acceptably small circles of confusion on the focal plane.
Acceptable, in this case, might be construed as a subjective judgement (dependent upon a viewer's visual acuity and viewing distance) but convention has defined it to be (at the focal plane) 0.00058 or 1/1720 of the focal length assuming that a viewer's distance is that at which a "proper perspective" is obtained. Thus, the acceptable subtended angular size of the circle of confusion of any image is dependent upon both the image size and the viewing distance.
Returning to the camera, the nearest point sharply defined in front of the focal plane is
(d^2*tanC)/(t+d*tanC)
where d=the distance focused upon, C=the angular size of the circle of confusion at 1/1720 of the focal length and t=the effective diameter of the lens (=the focal length divided by the f/number). Likewise, the farthest point sharply defined in front of the focal plane is
(d^2*tanC)/(t-d*tanC).
DOF=the distance between these two points.*
* These formulas are from the Van Nostrand's Scientific Encyclopedia, D. Van Nostrand Company, Inc., New York, 1968. Other sources may be more detailed but these will do for educational purposes, at the very least.
Note that no mention is made here of the size of the focal plane. That's because this is accounted for by the assumption that any real lens used in the calculation will have a sufficient FOV, behind the lens, to cover the film placed at the focal plane. So plug in your various film, focus distances and sensor sizes using any consistent number system and have at it! All the answers are here!
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My interest in this thread began with a comment from BJL who suggested the smaller format digicam has greater potential than we sometimes imagine. Here is a quote that caught my attention.
Indeed, we have wandered a very long way from that initial comment of mine, which was about low light capability, not primarily about depth of field. And even further from the original subject of this thread, the Leica Digilux 2.
I propose starting a new thread for anyone still discussing this. In order to attract Ray in particular, it will include arguments as to when and why we CAN expect some of the new, smaller digital formats to continue to offer less in the way of large apertures at a given angular field of view than 35mm format, and hence less latitude for extremely shallow depth of field and for fast, low light photography, even though 35mm currently does better than any larger format in these respects.
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BJL, What are you talking about, you’re on some mission but what is it!
You can get the photo with any camera, Leica, Rollieflex, MPP exposing a 30th at f5.6 and get a publishable photograph, if the focal length of the lens is the same you’ll even have approximately the same depth of field. The light making the image is the same regardless of image size. Ergonomics is another story!
If, however you want the subject to have the same percentage size in the image, you’ll need to use different lenses and for the same depth of field range a different aperture. If this is what you have been banging on about, it is the wrong reason to choose your format. The selection of format should be based on the purpose of the produced photograph and what determines acceptable and your own standards of image quality.
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One should also bear in mind that, by coincidence, 35mm is also very close to the length of the image so the term does not seem totally absurd even if one were not aware that it actually referred to the total width of the film.
2/3rds on the other hand seems to have no connection whatsoever with any dimension of the 2/3rd's format
Ray, that is quite a stretch: a format is designed with a frame 18x24 mm, diagonal 30mm, with the film width increased to 35mm by then need for sprocket holes on movie film; Leica doubles up the frame to 24x36, and by pure coincidence, 35mm becomes a "reasonable description"!
The dimensions like 2/3" (based on old vidicon tubes for TV, from long before silicon chips) do have a clear connection to sensor dimensions that is more than a coincidence; they are approximately proportional to sensor size, so one can quickly compare relative sizes, for instance noting that a 4/3" sensor has about twice the linear dimensions and four times the area of a 2/3" sensor.
The relation to actual dimensions is not so horrible either, for the few of us who actually care about such internal details; the height, width, and diagonal are about .4, .5, and .6 of that dimension; multiply by 10, 13 and 16 to throw in the metric conversion too.
About manufacturers wanting to hide the small sensor size, I would note on the contrary that in the mass market, small is cool. Many digicams are heavily marketed on the basis of being small, and Fuji boasts about efforts to make sensors smaller. Why would or should most customers be either surprised or bothered by the fact that their small camera contains a small CCD (if they even know what a CCD is, or that the camera contains one), so long as they are happy with its performance? This is on a par with thinking that the average casual photographer is bothered by getting more depth of field. Things are very different in the mainsteam than up here on Mount Olympus, or Mount Canon.
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I agree on the LCD, are they doing the hood again?
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Yes, got me there, I was looking at another digicam... 2/3rds a silly term really - for them that still use inches
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To what important photographic dimension does "35mm" refer to?
The width of the film.
Bruce
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DPReview just posted images of the Leica take a close look at the front element it looks most odd. Anyone seen this before?
Hi res images (http://www.dpreview.com/news/0312/digilux2p2/allroundview.jpg)
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I saw this earlier today also. I was so excited until my stomach flipped when I read electronic viewfinder. At least it is 100% coverage. Leica listed this camera on their web site at:
http://www.leica-camera.com/digitalekamera...x2/index_e.html (http://www.leica-camera.com/digitalekameras/digilux2/index_e.html)
I wish it listed how fast from power on until the first shot, and how fast of a shutter delay it had from squeezing the shutter release.
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The point is that when people skip all the math and skip all the theorizing, they end up concluding something stupid like DOF depends on sensor size when it does not.
In your example, the improved DOF you see is due to the smaller sensor using a much smaller physical aperture. It is not caused by shorter focal length!
It does not hold true in the real world that DOF effectively depends on sensor size. It depends on magnification and aperture. To fix your example, if I shot the scene with 35mm f/8 and 1/250, then shot the same scene with with the 4x5 and appropriate lens at f/28 and 1/60, the DOF for the 4x5 would be marginally better despite the fact that the aperture was identical.
You need to understand that holding the f-number constant across formats is meaningless and bogus. Once you figure that out, you'll realize the DOF claim is wrong.
Small sensor cameras require less light to expose them so they get by with smaller physical apertures and deliver correspondingly good DOF. If it were a 5050 you were shooting in the above example, f/8 would be the smallest aperture available. Why? It's diffraction limited at f/8! Shooting a 5050 at f/8 is like shooting your FF 35mm SLR at f/32 all the time (except that your 35mm would provide better DOF).
Now let's talk macro. With macro you are typically trying to maximize DOF (or at least make sure it's enough). If you are suggesting that a 5050 provides better DOF for a macro composition that FF 35mm you are sorely mistaken. At higher magnifications especially, 35mm will trounce the 5050. Do the math or better yet, take the pictures and you will see. Do that experiment, Paul, it might make you blue in the face.
OK, "F-Stop" numbers are dependent on focal length and physical aperture (and it is a non-obvious equation depending on exactly where the actual physical device is located). So saying F/8 for one lens and F/8 for another (different focal length) lens already accounts for the difference.
Why is it so stupid? You seem to be quite passionate about this, yet you miss the fundemental point: To get the same DoF for the same Field of View (FOV) for the same speed sensor, you get to use a higher shutter speed on a small format camera. Or, if you have a specific shutter speed in mind, you get more DoF on the smaller format camera.
Note that a 35mm is diffraction limited somewhere around F/18. And probably has about the same DoF for the same FOV.
And yes, I have shot macro on a 4x5. Let me tell you, it is not pleasant. Recommended method (by reputable sources such as "View Camera Technique") to get more DoF? Use a shorter focal length and crop; i.e. use a smaller format.
And yes, if the lenses were equal quality and equally corrected for macro, I would say a small sensor will out-perform a larger sensor.
And the point of all this is usually trying to get shallow DoF, where a small sensor camera can be very troublesome because F/2 on those short focal lengths still gives a lot of DoF. And yes it is because the physical aperture is smaller, but who cares? Same outcome either way. Getting all caught up in physical aperture sizes really doesn't change the outcome; too much DoF.
And I have done the math.
Since neither of us will move from our positions, my end of the discussion is finished. I agree to disagree.
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Dear me! I don't quite know what to say, Craig! :D What sort of nonsense are you trying to perpetrate! ???
The F stop is one of the great concepts of Photography precisely because it is[/i] meaningful across formats.
Meaningful only for exposure purposes! It is not the same as aperture!
A 300mm lens at f8 will always require the same shutter speed in the same lighting conditions whether that lens is attached to a 35mm camera, a tiny digicam or a 12"x16" field camera, and it will always produce the same DoF from the same shooting position.
No one cares about DOF when you've made no attempt to hold composition constant. If what you are saying is that f/8 delivers constant DOF regardless of camera, format, or focal length so long as the camera doesn't move (or the photographer doesn't) that's obviously nuts. Why don't you talk about different formats with matching perspectives since that's what everyone will ultimately be interested in. We take pictures of subjects, not just pieces dependent on what our formats allow us to have.
I've already mentioned in my response to Victor that 'only' changing the sensor size does not affect DoF. But you must realize that nobody designs and manufactures camera lenses without regard to the sensor size, film size, ie. format of camera the lens will be used with. However, it's true that lenses designed for a larger format can be and sometimes are used for a smaller format, the obvious example being the recent crop of DSLRs such as the D60, 10D and 300D.
So let's look at what happens when we take identical shots from the same position with a 50mm lens on a 1Ds and 10D. If the lens is the same and the F stop is the same and the distance to the central subject is the same . . . it follows that the exposure will be the same and the DoF will be the same. That's the beauty of the F stop system.
Actually it is the result of absolutely nothing changing other than the crop. It has nothing to do with the "f-stop system" and anyone can see that. The optics and shooting geometry are unchanged so it's no surprise that the image rendered on the film plane is identical. All that's different is how much is recorded.
But something will be different, won't it? The 10D shot will be severely cropped. In oder to get the same angle of view with the 10D I'll have to step back a few paces. Consequently, the DoF will now be greater on the 10D shot because I've increased the distance between camera and subject. Alternatively, I could just whack on a 28mm lens to get the same field of view from the same position. That would also have the effect of increasing DoF for the 10D shot (at the same aperture).
In the first case you've reduced magnfication. In the second case you've also changed the physical aperture assuming you meant "same f-stop". The actual focal length would be 31mm, so the second case is just theoretical. If you adjusted the f-stop to account for the change in focal length (same physical aperture) then DOF would not increase.
Now you could argue that the increased DoF that the smaller format 10D seems to exhibit has nothing to do with its smaller sensor size and that the real reason for the increased DoF is the fact that you either (a) had to step back to get a wider FoV or ( had to use a wider angle lens.
I would argue that in the first case but not the second. Focal length is not a determiner of DOF, aperture is.
However, there are reasons behind reasons. If the 10D did not have a smaller size sensor than the 1Ds, there would be no reason to step back or to fit a wider angle lens. In this example (which extends across all formats), the size of the sensor could be viewed as the more 'primary' cause of the increased DoF.
Except that now there would be no difference in DOF because the original persectives match. There would no longer be a reason to step back or change the lens. The only reason one could argue that the theoretical FF 10D would have better DOF in this case is that one could claim it had a larger COC due to its lower resolution. Clearly, two full frame 35mm SLRs with identical lenses, exposure settings, and perspectives deliver identical DOF when it's measured the same way.
Even from a purely mathematical point of view, there is a connection between sensor size, F stop and aperture if we're talking about standards lenses for the different formats. F stop is a relationship between the focal length of a lens and the physical dimensions of the aperture (F=FL/D). The focal length of a standard lens (as opposed to a telephoto or wide angle lens) is given by the diameter of the image circle the lens throws on the sensor. The size of the sensor or format bears a very direct relationship to the image circle. All 35mm lenses for example are designed to have an image circle a bit larger than the diagonal of the 36x24mm frame (about 43mm). The diagonal of an 8"x10" plate for a field camera is about 12.8"= 320mm. It's no accident that the standard lens for an 8x10 field camera is just that . . . 320mm.
Get my point? :)
Well, no I don't get your point. Different format sizes require different focal lengths for matching perspectives. Is that your point? I think we all know that. You are, however, mistaken about DOF and f-stops. You must change the focal length when you change format to match the perspective. When you do, you must change the f-stop so that the physical aperture remains the same. Then the DOF and perspective will match. The exposure will be off, but that's due to the change in exposure area of the sensor. Assuming like ISO, you will then have to adjust shutter speed. If you choose to leave the f-stop the same, DOF will change but it is due to the change in physical aperture, not the sensor size.
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Yes I did. You tried to say that F stop and DoF considerations do not apply across formats. When I challenged you on this point, you then wrote that it only applied to exposures and it was nuts to pretend that a constant DoF across formats was relevant because people do not take photos of bits and pieces but whole compositions (or words to that effect).
I did not. I said that fixing f-stops while varying focal lengths across formats was invalid if you were trying to compare DOF of the two formats. In fact, what I said was that the resultant decrease in physical aperture (due to fixing f-stop) was the cause of the improved DOF, not the smaller sensor size itself. You did not challenge this statement. The rest of your quote just proves that you didn't understand what I was saying at all.
What is nuts or irrelevant, or what constitutes a bit or a piece or a whole composition, is a matter of opinion; a matter of taste and a matter of esthetics. I'm trying to cut through your obfuscation with some facts.
That a 300mm lens at a given f stop will have the same size aperture and the same DoF (from the same shooting position)[/I]irrespective of the format the lens was designed for is surely of some academic interest to those who didn't already know it, and is surely of more than academic interest to owners of APS format DSLRs who didn't already know it.
I'm not sure how this ties into the discussion, but it's true that if you fix the focal length and f-stop, DOF will be the same regardless of format given identical CoC.
You seem to be looking for direct causes.
Of course I am! What directly controls DOF is all that matters in this discussion. Sensor size does not determine DOF. The sensors themselves only record the image rendered on them! DOF is in the optics (though sensor size effects magnification for a desired perspective).
Are you trying to say that only direct causes are valid? A lot of things that happen are a consequence of a consequence of a consequence etc. Exponents of Chaos Theory will have us believe that a butterfly flapping its wings in South America might just tip the balance and cause a storm in Australia (I wish it would. It's still pretty dry over here.) This is an extreme example and I don't take it seriously. That reducing sensor size might (and certainly will if you keep the f stop and FoV the same) result in an increase in DoF has a much more direct causality.
Look who's obfuscating now!
By the way, it's difficult to hide a phrase like "same f stop" in such a short sentence. I don't know how I managed that.
If the educated photographic community were to support the notion that smaller formats offered inherently better DOF only because it's indirectly connected or observed to be true in typical usage, then such incorrect assumptions would lead to the belief that small formats are superior for macro (untrue) or small formats are unable to limit DOF (untrue). How specific examples of a format perforrn is irrelevant to the inherent capabilities of that format.
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Alright Ray, I'll enlighten you assuming it's possible.
I did not. I said that fixing f-stops while varying focal lengths across formats was invalid if you were trying to compare DOF of the two formats........
This reminds me a bit of the sorts of arguments I used to have with my ex wife. "But you said .... "No I didn't, I said...." and so on. I wish I'd had a tape recorder running continuously. :)
We do have a recorder running. but you chose to paraphrase (incorrectly) instead of quoting since it suited your purpose.
Fortunately on this forum we have a record of everything written. This is exactly what you wrote:-
You need to understand that holding the f-number constant across formats is meaningless and bogus. Once you figure that out, you'll realize the DOF claim is wrong.
Thank's for removing that sentence from its context, Ray. That statement was most clearly about DOF, not exposure, and was entirely correct. Interesting that you can find a quote now since it suits you.
This statement is clearly, definitely and demonstratively wrong.
Holding the f-number constant across formats is neither meaningless nor bogus. The physics that determines that DoF will remain the same for a lens of a given focal length and given f stop irrespective of camera format, is the same physics that determines that an increase in DoF will result if the focal length is shortened and the f stop kept constant. Every photographer should know this.
Please Ray. You have no intention of holding focal length constant (or anything else for that matter). The thrust of your endless arguments is that you reserve the right to vary every parameter as it suites you, then claim that small sensors have a DOF advantage. Why don't you settle down and pick a single, unchanging, meaningful test so that progress can be made? Are you afraid of the consequences?
As I've said many times, you cannot change sensor size and focal length (leaving f-stop constant) then claim that the DOF change is due to the sensor size. It is due to the change in physical aperture and change in the acceptable parameters of the shot which may not be obvious to some people. If you want to discuss DOF, then talk aperture. If you want to talk exposure, then talk f-stop. The two are not the same, and your dogged commitment to keeping them confused is just an attempt to skirt the issue.
It is also sometimes of practical significance to LF photographers who use a shift mechanism for architecture shots. As I'm sure you already know, to use tilt and shift requires that the image circle is significantly larger than the film format. In order to achieve this larger image circle, LF photographers will sometimes use a lens designed for a larger format (say 8x10) with a smaller format body (say 4x5). They have to be aware that in doing this, DoF does not change for the same f stop. This is not meaningless or bogus.
Now you might argue that this is obvious, and I would agree that it is obvious to those who have thought about it and already know it. But it's not necessarily obvious to those who have not thought about it.
In any case, I wasn't aware this discussion was about the relative transparency of various statements. None of my statements in this thread are incorrect (as far as I know. If any are, please enlighten me :) )
As I recall (and there's a accurate record to back me up), it was you who first accused me of trying to "perpetrate nonsense" then accused me of deliberately obfuscating the issues. Now all you offer is specious comments about chaos theory and specialized lenses, as if any of that is necessary or valuable to the argument. You are attempting to shift the argument to a discussion lenses of a fixed focal length even though I've never mentioned it and it has nothing to do with the original argument. It is something you can get right, though. Thanks for the clarification.
If you aren't enlightened at this point, I'd suggest you take on BJL's proposal. It is thoughtful and reasonable. It is curious that you believe that your artistry is forced to change because you vision of a shot is hopelessly tied to the format. I believe in reality that the conclusion is inescapable once you agree to play the game.
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Thank you Leon, some sanity at last. If you review the article notes on technical aspects of photography (http://www.bobwheeler.com/photo/Documents/documents.html) I posted earlier, I you will find your reference to Circle of Confusion. Of course we may both be missing the point, it maybe just about size
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... Enter a nice gentleman like yourself who hands me a Leica Digicam; do I now use F2 at 1/250th? Almost certainly not. I might well stick with the same f stop and shutter speed, thus lifting those constraints, achieving greater DoF and a more appropriate shutter speed for a hand held shot with the shorter lens.
Thank you Ray; your example shows that in this situation, the smaller format probably gives you more flexibility; in this case by making it easier to achieve more depth of field in a limited light situation. This corroborates my comments about smaller formats having an advantage in low light situations. You are almost saying that with the larger format, you might like to get more DoF than F/5.6 gives you, but are forced to a less satisfactory compromise by the limitations of that equipment (superable if you had sufficiently fast film or sensor.)
And as you say, the struggle to achieve more DoF is probably far more common than the struggle to achieve less, so to the extent that this is true, smaller format cameras might have some advantage (but at the cost of lower image quality in other ways).
P. S. Ray, I just read your further comment on this after replying to your earlier post; maybe we are now all agreed that smaller formats are likely to be better off, or at least not worse off, in typical low light situations?
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To what important photographic dimension does "35mm" refer to?
The width of the film.
Bruce
Right of course: the total film roll width, including space for sprocket holes and maybe the movie sound-track. For obscurity and irrelevance to digital photography, this is on a par with the "diameter of a vidicon tube" explanation for sensor dimensions; which makes me wonder why I so often read people addressing scorn at the latter and none at the former. Since this happens a lot in the US too, it is not just the use of inches!
P. S. On the subject of strange dimensions, there is also Canon saying things like "The EOS 300D features an APS-C sized sensor". Actually APS-C is 25.5x17mm, not the 300D's 22.7x15.1mm, so going from the vague APS to a name that refers to specific, wrong, dimensions puzzles me.
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This 2/3" is because electrical engineers did it that way! It is odd 11mm is almost half an inch at 25.4mm where as 2/3 would be 16.93mm.
This summs it up -> DPReview - Making (some) sense out of sensor sizes (http://www.dpreview.com/news/0210/02100402sensorsizes.asp)
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OK, I will play the devils's advocate, and suggest that the Leica Digilux 2 (aka the Pansonic something-or-other) might at least be a succesful "concept camera" for a certain photographic niche. Like a concept car, it will not necessarily sell in large quantities though, or at all.
Apart from the appeal to some of us of a compact digital camera that allows one to control focus, aperture and focal length with rotating wheels on the lens barrel, the big debate will be over sensor and pixel size, and the EVF. I will leave the latter to another post.
The key here is noting that smaller formats can typically be used at lower f-stop's, and hence at lower ISO settings, off-setting their weakness when compared at equal ISO, and that bigger sensors can only provide their potentially greater image quality when provided with enough light to avoid underexposure.
Larger sensors can give higher image quality when provided with enough light, but an "information theoretic" approach suggests that with a given, limited amount of light, any sensor big enough to take in all the available light without highlight blow-outs, combined with a fast enough lens, can give "image detail" as good as a larger sensor, which has to be unerexposed with that same light levels. Below a certain moderate level of total available light, even 2/3" format can roughly match any larger format. However the lens speed of the Digilux 2 is not enough to fully take advantage of this: I dream of a future f/1 zoom lens in this very small format.
My measure of "detail" is roughly resolution (pixel count) times tonal detail at each location (S/N ratio). If the same total amount of light is gathered (same exposure duration, same aperture diameter and hence f-stop in proportion to focal length and sensor size), sensors of different sizes with the same number of photosites will each get the same amount of light per site, but the larger sites will have more thermal noise, so worse S/N ratio. If the larger sensor instead has more photosites of the same size as the smaller sensor, each photosite gets less light, so again worse S/N ratio, balanced against higher resolution. Downsampling to the resolution of the smaller sensor will return one roughly to the situation of both sensors having the same photosite count; the smaller one still wins on noise.
For the Digilux 2 used at its maximum of f/2 with a shutter speed of 1/100, the sensor can handle all available light at about EV 9 or below (6 stops or more below bright sunlight), rising to EV 10 if 1/200 is needed and so on, so for a larger sensor to have an advantage in light this low or with shutter speeds at least this fast needed, it can only do it by going to larger aperture diameters and hence less DOF; as far as DOF, the equivalent f-stop is about f/5.6 with APS DSLR's. Allow an APS DSLR a zoom lens going down to f/2.8, and the threshold where the Digilux 2 is at no disadvantage goes down to EV 7 at 1/100, EV 8 at 1/200 and so on. Optimal f-stop for image quality seems to be smaller; around f/11 in APS format, and using that and the Digilux 2 at the equivalent f/4 pushes the break-even to about EV 11, just four stops below bright sunlight.
As to the "concept camera" idea, the potential is that (a) the maximum usable light level per pixel might be increased by several stops by ideal like latest Fuji's SuperCCD design, which would increase by several stops the light level needed before a larger format has any advantage ( in the very small 2/3" format, a f/1 lens should be no harder to make than f/4 for comparable FOV in 35mm format, and that would add two stops of high shutter speed potential © the resolution can be increased; Sony has already announced 8MP in 2/3" format, and that is more resolution than the overwhelming majority of prints need (being significantly more than given by any APS format DSLR!)
Finally, do not be mislead by the rather common claim that more, smaller pixels on a sensor of the same size must give worse noise in an image: smaller pixels in the same quantity on a smaller sensor do that, but increasing pixel count in proportion to pixel size roughly balances out the noise level seen in a print of a given size, and also balances any apparent loss in fineness of tonal gradations and range, all through the extra "dithering" or "averaging" effect of printing more, smaller pixels. A print can have far wider dynamic range than the individual pixels: consider half tone printing where the many tiny printed pixels are pure black or white, or consider the pieces of black metalic silver on a traditional print as "1-bit pixels".
In summary, I think that there is a reason why sensor makers are moving almost uniformly towards more, smaller pixels despite the horror often expressed on the internet at this trend, and I think this is the explanation.
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However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
That's because when you adjusted the focal length, you changed the physical aperture to keep the f-number constant. DOF didn't increase because of the smaller sensor, it increased because you closed the physical aperture!
Um yeah, true. What is your point?
I don't get why people argue this over and over and over again. Skip all the math, skip all the theorizing. Try this:
Take a small sensor camera. Make it a P&S, a 35mm, what ever. Select a 3D scene. Select an exposure; lets say 1/250 @ F8 @ ISO 100. Take picture.
Now take a 4x5. Insert ISO 100 Film (or a digital back if you are quite well off). Select focal length for the same composition from the same position. Select the same exposure. Do not use movements. Take picture.
Print both at the same size; lets say 8x10. Visuallly inspect. Do they have the same DoF? I can assure you it is not even in the same ballpark. The DoF in the 4x5 will be very very very thin. The P&S DoF will be very good.
Now argue all you want on why, but it still holds in the real world: DoF EFFECTIVELY depends on the sensor size. You can blame it on focal length if you want, you can point at this variable or that changed, argue to you are blue in the face, but you will still have more DoF for your desired composition on the small sensor camera.
The point is that when people skip all the math and skip all the theorizing, they end up concluding something stupid like DOF depends on sensor size when it does not.
In your example, the improved DOF you see is due to the smaller sensor using a much smaller physical aperture. It is not caused by shorter focal length!
It does not hold true in the real world that DOF effectively depends on sensor size. It depends on magnification and aperture. To fix your example, if I shot the scene with 35mm f/8 and 1/250, then shot the same scene with with the 4x5 and appropriate lens at f/28 and 1/60, the DOF for the 4x5 would be marginally better despite the fact that the aperture was identical.
You need to understand that holding the f-number constant across formats is meaningless and bogus. Once you figure that out, you'll realize the DOF claim is wrong.
Small sensor cameras require less light to expose them so they get by with smaller physical apertures and deliver correspondingly good DOF. If it were a 5050 you were shooting in the above example, f/8 would be the smallest aperture available. Why? It's diffraction limited at f/8! Shooting a 5050 at f/8 is like shooting your FF 35mm SLR at f/32 all the time (except that your 35mm would provide better DOF).
Now let's talk macro. With macro you are typically trying to maximize DOF (or at least make sure it's enough). If you are suggesting that a 5050 provides better DOF for a macro composition that FF 35mm you are sorely mistaken. At higher magnifications especially, 35mm will trounce the 5050. Do the math or better yet, take the pictures and you will see. Do that experiment, Paul, it might make you blue in the face.
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Craig,
You're not telling me anything I don't know. I don't understand where you got the idea that I think F stop is synonymous with aperture diameter. What you are doing is engaging in obfuscation, perhaps because you don't like the method by which I've explained things; perhaps just to show off your knowledge.
Nothing you've written so far provides any evidence or reason why what I've written on this subject is wrong. What you seem to have done is draw an incorrect inference from some of my statements and are now arguing against that incorrect inference. I can't be responsible for people misunderstanding what I say. All I can do is try to be as clear as possible.
So let's go back to what I actually wrote.
However, if you want to maintain the same angle of view from the same shooting position, DoF increases, for any given f stop, as the sensor size decreases.
This statement is absolutely true. I can't fault it. I have not said that sensor size determines DoF. If you think that's what I meant, then it's you who has made an incorrect inference. DoF is never determined by one factor, not f stop alone, not physical aperture alone not focal length alone not sensor size alone. But sensor size is[/i] going to determine the focal length of the lens required for a certain composition shot from a certain position.
Perhaps I could have provided a fuller explanation. Isn't this always the case? I could have tacked on the obvious explanation that the reason DoF increases, at a given f stop, as sensor size decreases, is that to maintain the same field of view it's necessary to use an increasingly shorter focal length lens. There's a relationship between sensor size, lens focal length and Field of View. There's also a relationship between focal length and physical aperture size. It's called F stop.
I really don't know what you are arguing about.
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Ray,
you seem to have completely misunderstood my question, which in particular was NOT about deciding which format to use for a certain situation. Let me try again. Suppose that you have set up to take a photo with a certain format, and have chosen your camera position, focal length, aperture setting, ISO, shutter speed etc. to give what you consider to be the most desirable artistic result, with no consideration given to mundane issues like the size, weight, cost or availability of the lens needed.
Then you are asked to do it instead with a camera of a smaller format, but with an equally unlimited lens options. How would you change your "ideal" choice of lens and settings?
My expectation is that, with the same artistic objective, you would want to keep the same perspective, depth of field, and either want the same shutter speed or be open to any higer speed too, and so would keep the same camera position, adjust focal length in proportion to image sze, keep the same aperture diameter, and so use a proportionately larger f-stop. Speed choices are less clear; perhaps the same ISO and thus the indicated higher shutter speed, perhaps keep it the same or closer to what you chose for the larger format with a lower ISO setting or possibly an ND filter.
If not, please explain why you ideal, preferred composition would instead change in perspective, depth of field or such.
P. S. you allegation about my lens envy is quite bizarre, since you are the only one bringing your available lens limitations into the discussion of compositional choices.
Quite sincerely, my biggest single reason for having balked at buying a Canon 10D (which would work with my existing Canon 28-105 lens and a few accessories) is the poor lens choices for the type of photography that I do.
Conversely, my main reason for currently favouring the Olympus E-1 is that its 14-54 f/2.8-3.5 is by far the most attractive current or announced choice for my particular purposes, offering FOV comparable to 25-100mm in 35mm format for the traditional print shapes I prefer; 4x5 to 3x4. Comparing to the 10D, the comparision is about 16-63 for FOV, between f/2.8-3.5 and f/4-5 for speed (the slower range allowing for the extra stop or so of usable pushed sensor speed on the 10D), f/4-5 for minimum depth of field.
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Let me give you a concrete example from recent experience. Many art galleries, museums, historic homes etc have rules about photography. Sometimes no photography at all is permitted. Sometimes photography is permitted but no flash or tripod allowed.
In such circumstances one struggles with the competing constraints of shutter speed and DoF. One usually ends up with a compromise, or no shots at all; perhaps f5.6 at 1/30th is the best one can do.
Enter a nice gentleman like yourself who hands me a Leica Digicam; do I now use F2 at 1/250th? Almost certainly not. I might well stick with the same f stop and shutter speed, thus lifting those constraints, achieving greater DoF and a more appropriate shutter speed for a hand held shot with the shorter lens.
This is a good one. You choose a low light situation, then extoll the virtues of the more light-sensitive camera. Of course, we get it but how does that matter in the discussion?
The point of BJL's scenario is to free you from any concerns that may limit your artistic choices. It is then that you can freely choose to duplicate your photographic choices in another format. Once you do that, you will see that DOF is not dependent on sensor size.
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I can't believe this: five (5) pages of supertech mumbo-jumbo hyperbole about DOF without a single reference to the formula or to the almighty critical role of the "circle of confusion"!
Yes, it's weird isn't it! I get the impression some people are just arguing for the sake of arguing, and it's probably now time for me to bow out of the discussion.
My interest in this thread began with a comment from BJL who suggested the smaller format digicam has greater potential than we sometimes imagine. Here is a quote that caught my attention.
The key here is noting that smaller formats can typically be used at lower f-stop's, and hence at lower ISO settings, off-setting their weakness when compared at equal ISO, and that bigger sensors can only provide their potentially greater image quality when provided with enough light to avoid underexposure.
Larger sensors can give higher image quality when provided with enough light, but an "information theoretic" approach suggests that with a given, limited amount of light, any sensor big enough to take in all the available light without highlight blow-outs, combined with a fast enough lens, can give "image detail" as good as a larger sensor, which has to be unerexposed with that same light levels. Below a certain moderate level of total available light, even 2/3" format can roughly match any larger format. However the lens speed of the Digilux 2 is not enough to fully take advantage of this: I dream of a future f/1 zoom lens in this very small format.
I was really mainly interested in exploring this idea but in the process seem to have upset, enraged and confused some people by indirectly associating format size with DoF.
I don't generally find it useful in my photography to calculate the diameters of 'circles of confusion' or the physical size of apertures at a particular f stop, but thanks for your formulae. I'm sure some people will find them useful. :)
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For example, the DOF of the "cropped" image from a Canon 10D with a 50mm lens focused at 3m at f/8 is exactly the same as the DOF of the "full" image from a Canon 1Ds with the same lens, focus setting, aperture and camera position. Of course, the image content from the 1Ds is not the same because the "crop" is gone, but the DOF of that wider image will not change because none of the original DOF variables will have changed!
That is so if you keep the circle of confusion the same. This is turn might make sense if you make a print in which the same subject in the image comes out the same size on the print, which is then viewed from the same distance. That way, the circles of confusion as they appear on the print are the same size, and with the same viewing distance, have the same effect on perceived sharpness.
But if you make the same size print for the "cropped, mildly telephoto" 10D image as for the "normal FoV" 1Ds image, the greater magnification from sensor to print will make the printed circles of confusion bigger, and so give less DoF in the image from the 10D than in the image from the 1Ds. The hitch is that the "angular CoC" in your formulas assumes a lens of "normal" angular FoV, which the 50mm ceases to be when used on the 10D.
To maintain the same DoF in this second comparison, the DoF scale for the smaller format needs to be computed on the basis of a smaller CoC.
(There is more about this in my very long new post in a new thread, though I omitted all formulas, hoping that everyone here understands their main conseqences by now! That might not be fair to newcomers who missed the previous long DoF discussion.)
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One should also bear in mind that, by coincidence, 35mm is also very close to the length of the image so the term does not seem totally absurd even if one were not aware that it actually referred to the total width of the film.
2/3rds on the other hand seems to have no connection whatsoever with any dimension of the 2/3rd's format, not the height, not the width, not the diagonal. So what we have here is tradition supplanting common sense.
I suspect the real reason the term is still used is to obscure the fact that a 2/3rds sensor is really very tiny. It's not good for marketing purposes to let it be widely known that the imaging chip in the camera you're flogging has only 1/16th the area of the standard miniature (35mm) camera that people are familiar with.
It would be no exaggeration to describe these point and shoot digicams as sub-miniature cameras.
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True! On the other hand, if you go back to the beginning of this thread where there's a link to the relevant page on the Leica site, you'll find the following statement:-
The 2/3-inch CCD sensor is very large for its resolution of 5 million pixels.
.... and there's no mention of the actual dimensions of the 2/3rds format. In fact it took me quite a bit of searching on the net to find those dimensions. Of course, I ended up at dpreview where I should have gone in the first instance.
I'm not talking about conspiracy theories here but rather a disincentive to get rid of a meaningless term because of its marketing advantage. I'm sure there are lots of potential buyers out there who equate pixel count with quality and resolution and who are attracted to the idea that such a small 5MP camera has almost as many pixels as a Canon 300D.
The only meaning I could get out of the brief dpreview explanation is that 2/3rds expresses the relationship between the sensor size and the size of the image circle thrown by the lens. Now that's really useful information to have. :D
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A bit more briefly, about the EVF, since this is a developing concept that the Leica Digilux 2 will put to the test. For this, one has to accept the idea of using a sensor in 2/3" format or smaller; this should not be so hard since the overwhelming majority of digital cameras have such small sensors, including some that have produced quite good and even professional images.
Given that the sensor is so small, the EVF is probably already the best option, or will soon be: consider the alternatives.
a) peep-hole viewfinders, including true range-finders. These have poor compatability with wide ranges of focal lengths, from either ultra-wide ranging zooms, inerchangable lenses or use of supplementary lenses on digital cameras. None currently works with sup lenses since the sup lens blocks the viewfinder, and as Lecia has pointed out, once you put a sufficiently big fast lens on the camera, the peep-hole VF again gets obstructed. They also lack depth of field preview, and various other "TTL" virtues.
LCD on the back. The problems are well known: they require holding the camera in an unstable "arms' length" position, and give a small image that is hard to see in bright light.
c) Optical TTL VF. The image is currently constrained to be rather small or dim or both with such small formats. If the image were made as large as in 35mm format VF with the f/2 lens of the digilux, brightness would be as for an f/8 lens in 35m format. The mythical f/1 zoom that I fantasize about in my previous post would improve this to a tolerable but not great f/4 equivalent brightness. That would still leave the other disadvantages of SLR's, like bulk and noise.
Compared to all this, a high resolution EVF's with zooming at the focus point to aid manual focus are probably the future for high end compact digital cameras. Actually I believe that every current 2/3" format digicam has an EVF except the Olympus E-20 which is approaching end of life, so I am not sticking my neck out much with that prediction.
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Ray,
>I see you haven't lost your enthusiasm for the smaller format.
Not for "limited light photography" as I envision for cameras like the Digilux 2; and the dominance of 35mm over all larger film formats for low light and high speed photography shows that I am far from alone.
>The smaller the format, the smaller the maximum print size in
>relation to a given standard of acceptable sharpness and the
>narrower the range of DoF settings (for acceptable sharpness).
>I see no way around this.
Agreed, with one crucial qualification: shifting to a larger format only fully gives this advantage when it allows one to gather more light, through some combination of larger aperture diameters (less DoF etc.) and longer exposure times that would not possible in the smaller format because it would lead to overexposure.
In "signal to noise ratio" terms, a larger format benefits when it gathers more signal, but if it is limited to gathering the same signal as a smaller format, gathering that light over a larger sensor area collects more sensor noise along with the same signal, and so it is worse off. The larger sensor is not any worse off though, since one can instead use the same focal length as with the smaller format, forming the same size of image, and then crop, discarding the extra noise at the unused photosites.
Only cost and weight constraints, and perhaps sensor read-out speed, wil fundamentally prevent a larger format from performing at least as well as a smaller one.
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I guess you’ll all be shouting at me in a moment, but this stuff is photography 101! Reading through this ant the other thread on this subject gives me the impression that there’s some basic photographic foundation knowledge is missing, and in turn a lack of common vocabulary.
In the UK (a place known for good photographers) they teach this stuff with this http://www.amazon.com/exec....s=books (http://www.amazon.com/exec/obidos/tg/detail/-/0240512561/qid=1070611908/sr=1-5/ref=sr_1_5/102-9016544-6821769?v=glance&s=books) book - Basic photography by M. Langford. I would recommend it, so you can understand what effects the lights path from subject to the focal plane. It is also impossible to explain this without pictures of the light path.
And before you all get mad at me, remember it is only about the final picture, and it is not until the pictures are produced can we have an opinion on will this Leica be good enough or not!
Be happy, Victor
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Those people don't necessarily fail to realize that the physical aperture has changed, but perhaps they like shooting at 1/60.
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In the real world, a small sensor camera will have difficultly restricting DoF. Maybe they will release a F/0.7 lens on a small-format camera in the future with ISO 16 so you can both limit DoF and have a slower shutter speed, but I have my doubts.
Now we are getting somewhere.
I) Limitations of available lenses.
To take your second point first, I agree, with some qualifications:
a) Current digital cameras with sensors distinctly smaller than 35mm format have difficulty restricting DoF, due to their small actual maximum apertures, and for sufficiently small formats (2/3" and below) this might always be the case, if only for the market driven reasons that for small, convenient cameras, seeking more DoF is far more common than seeking less, so the makers' choice of maximum aperture is driven mostly by speed rather than DoF considerations: f/0.7 will never be in much demand.
Current "smaller than 35mm format" interchangable lens DSLR's are more limited in shallow DoF options at normal to wide focal lengths by current lens options. Again, it is likely that there is not enough demand for less DoF in wide angle, so aperture options wil be driven by speed needs, which are not very great in wide angle: the trend after all is f/4 only rather than f/2.8 even for high quality lenses, taking advantage of the greater usable ISO speed of DSLR's compared to film to reduce cost and weight.
On the other hand, into the telephoto range, the maximum aperture available does not increase much with focal length (or, the minimum f-stop declines: f/1.2, f/1.4, f/2, f/2.8, f/4 ...) so for equivalent FOV, DoF options are not much dfferent between APS and 35mm DSLR's, and both can generally get shallower than in MF or LF, where there is less in the way of telephoto lenses with huge maximum aperture diameters. Further, in this realm, more speed is always in demand, so I can easily imagine in the future something like a 300/2 as an APS counterpart of 400/2.8 in 35mm format.
II) Shooting at a given shutter speed.
Now to the first point, of wanting to shoot at a given shutter speed.
If this is for the sake of freezing subject of camera motion, it is only a lower limit on shutter speed, so the only effect is to force larger formats to give up some DoF; those issues to do not prevent the user of a smaller format increasing speed to compensate for the greater speed at a given DoF.
If the issue is having a long enough exposure time to produce some effect like deliberate motion blur, all it takes with the smaller format is some combination of changing to a lower ISO setting (which gives better image quality anyway) and if that is not enough, adding an appropriate ND filter.
Either way, nothing about shutter speed choice prevents the smaller format from achieving DoF as shallow as with a larger format; given appropriate lenses, smaller formats just have a few more options.
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I did not. I said that fixing f-stops while varying focal lengths across formats was invalid if you were trying to compare DOF of the two formats........
This reminds me a bit of the sorts of arguments I used to have with my ex wife. "But you said .... "No I didn't, I said...." and so on. I wish I'd had a tape recorder running continuously. :)
Fortunately on this forum we have a record of everything written. This is exactly what you wrote:-
You need to understand that holding the f-number constant across formats is meaningless and bogus. Once you figure that out, you'll realize the DOF claim is wrong.
This statement is clearly, definitely and demonstratively wrong.
Holding the f-number constant across formats is neither meaningless nor bogus. The physics that determines that DoF will remain the same for a lens of a given focal length and given f stop irrespective of camera format, is the same physics that determines that an increase in DoF will result if the focal length is shortened and the f stop kept constant. Every photographer should know this.
It is also sometimes of practical significance to LF photographers who use a shift mechanism for architecture shots. As I'm sure you already know, to use tilt and shift requires that the image circle is significantly larger than the film format. In order to achieve this larger image circle, LF photographers will sometimes use a lens designed for a larger format (say 8x10) with a smaller format body (say 4x5). They have to be aware that in doing this, DoF does not change for the same f stop. This is not meaningless or bogus.
Now you might argue that this is obvious, and I would agree that it is obvious to those who have thought about it and already know it. But it's not necessarily obvious to those who have not thought about it.
In any case, I wasn't aware this discussion was about the relative transparency of various statements. None of my statements in this thread are incorrect (as far as I know. If any are, please enlighten me :) )
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Please Ray. You have no intention of holding focal length constant (or anything else for that matter). The thrust of your endless arguments is that you reserve the right to vary every parameter as it suites you, then claim that small sensors have a DOF advantage.
Now you're beginning to make sense. Yes I do reserve the right to vary every parameter as it suits me, and it is perfectly true that small sensor cameras have a DoF advantage as well as an exposure advantage in low light situations. :)
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I don't generally find it useful in my photography to calculate the diameters of 'circles of confusion' or the physical size of apertures at a particular f stop, but thanks for your formulae. I'm sure some people will find them useful.
Nor do I, Ray. These things have already been done for us by the manufacturer's data included with new lenses, and we can often preview actual DOF within our cameras.
The point is that the only variables in the DOF formulas are the focusing distance, the diameter of the aperture and the size of the circle of confusion (which is a funtion of the focal length of the lens). Thus, these are the only factors that can possibly have any direct affect on DOF in any given configuration. Change the configuration, e.g., to a larger film format that requires a different lens, and one or more of these variables will have to change accordingly in order to change the DOF.
For example, the DOF of the "cropped" image from a Canon 10D with a 50mm lens focused at 3m at f/8 is exactly the same as the DOF of the "full" image from a Canon 1Ds with the same lens, focus setting, aperture and camera position. Of course, the image content from the 1Ds is not the same because the "crop" is gone, but the DOF of that wider image will not change because none of the original DOF variables will have changed! Simple, huh! ::
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BJL, Having done many years of professional action photography I have to agree that 35mm has a lot of advantages and delivers the quality. The only reasons not to use a format are quality and/or ergonomics.
IMHO the smaller formats extra depth may be more of a problem than a benefit. Seperating subject from some distraction is an often challenging requirement.
And f5.6 was just a choice, I'm often stuck on f2.8 for exactly the reason you describe - no light! Yet for most this discussion is just a big so what!
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But worse it is in inches why not mm!
The use of inches is anachronistic but not so surprising, being a product of American television engineers in the 1950's or so; it is more interesting to me that some decades earlier, Thomas Edison and George Eastman went metric for the 35mm film format. But no-one has answered my question about the significance of the length 35mm.
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That is odd. I've seen square lens hoods before, but nothing like that.