Luminous Landscape Forum
Equipment & Techniques => Cameras, Lenses and Shooting gear => Topic started by: gkramer on January 27, 2007, 11:08:19 pm
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Sidney Johnson's analysis of DoF ("Lens Equivalents") is based on a film-camera era treatment of the topic, and is not valid for digital. The basic problem is that it is implicitly based on the "enlargement factor" as measured by the ratio, in terms of linear dimensions, by which the film negative (or positive) is enlarged to to produce the final print (thus a 35mm negative, which measures 24 x 36mm = 1" x 1.5", when enlarged to produce an 8" x 12" print, has an "enlargement factor" of 8x).
This is inappropriate for digital: the relevant "enalrgement factor" depends only on the pixel count, and is idependent of the physical dimensions of the sensor. A 6MP, 2000 x 3000-pixel image file will produce the 6.7 x 10" print at "native" 300ppi resolution, irrespective of whether it was captured by a 24mm x 36mm full-frame sensor, a 16mm x 24mm APS-C sensor, or a tiny, quarter-inch sensor from a pocket point-n-shoot.
Johnson suggests setting the CoC to (sensor diagonal)/1500, which for a fullframe 24mm x 36mm sensor gives a CoC of 29 microns diameter. For the Canon EOS-1Ds, with a pixel pitch of 8.9 microns, this means the CoC on the sensor is 29/8.9 = 3.25 pixels in diameter; at 300ppi, this would be about .011" on the print.
The Nikon D2X has about the same MP count (12.2MP), on a smaller APS-C sensor, with a much smaller pixel pitch, 5.5 microns; but suppose Nikon decided to produce a full-frame DSLR by simply scaling up the D2X's sensor (it would be an impressive 27MP). Then the CoC on the sensor would be 29/5.5 = 5.27 pixels in diameter, and would produce a blur spot on the print 1.6 times larger, or .017". The difference on the prints (viewed under identical viewing conditions) would be clearly visible.
Of course the print produced by the 27MP Nikon D3Z (let's call it) would be much larger than the 11MP Canon's; at "native" 300-ppi resolution, the "D3Z" image would be 9.5" x 14.3", versus the Canon's 6.8" x 10.3"--roughly the difference between a double-page spread in Audobon magazine, and a shot that would fit comfortably on a single page. But, viewed at the same distance, the blur spot used to define DoF of the D2X image would be visibly larger; or, put differently, the apparent DoF of the D3Z image would be less.
To correctly calculate the DoF for the D3Z, we must use a smaller CoC, of 29/1.6 = 18 microns.
One implication of this is that the DoF scales engraved on lenses are meaningless when the lens is attached to a DSLR, since they do not take account of either sensor size or pixle pitch; and even tables like Johnson's, which do take account of sensor size, are off the mark. I'm working out a corrected one, which I hope to post later.
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Anyone with a basic understanding of DoF knows CoC is determined by the photogtapher, not the camera (or its maker) or sensor type or size or pixel pitch or kind of film being used, lens, or whatever. It is simply a number decided on by the photographer when designing a print.
It makes no sense of course to select a CoC smaller than can be resolved by the camera system, be it digital or film. There is a distinct difference between DoF and maximum DoF.
Sidney Johnson made some assumptions about the size of enlargement and viewing conditions in order to deteremine a CoC, but that CoC is valid only for his set of assumptions.
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Sidney Johnson made some assumptions about the size of enlargement and viewing conditions in order to deteremine a CoC, but that CoC is valid only for his set of assumptions.
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Give it a rest Howard. The DOF perceived by a viewer of a print depends amongst other things on print size and viewing distance, and so these conditions are a necessary part of any DOF calculation or comparison. These are "assumptions" only in the same sense that reference to a particular focal length, aperture ratio and focus distance are "assumptions": they are all just explicit parts of the statement.
And in fact, so long as one only assumes that different prints from different combinations of focal length, aperture, subject distance etc. are assessed via prints of the same size, viewed from the same distance, by a viewer of the same visual acuity, Johnson's conclusions hold: equal aperture size d=F/N, combined with equal perspective, equal FOV, equal image size and equal viewing distance will give about equal DOF.
Johnson does make some assumptions, mostly rather harmless. Firstly that we are not in the "macro" range, where some of the optical approximations made might become less accurate. Secondly that the sensor resolution is not an issue because it is more than the viewer can make out under the stated viewing conditions, and that lens aberrations are likewise low enough to be ignored.
It might be interesting to rework Johnson's calculations in terms of statements about the effects on angular resolution of various parts of the image from the combination of OOF effects (the various circles of confusion at different parts of the image), Airy disks, pixel sizes, etc. This is more the spirit in which astronomers approach resolution.
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Give it a rest Howard. ...
Johnson does make some assumptions, mostly rather harmless. ...
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No.
I didn't say Johnson's assumptions were not harmless or even unusual. I merely said that assumptions were made and that the reader must understand those assumptions in order to understand Johnson's results.
Not knowing or acknowledging assumptions often leads to results becoming "facts." Like, DoF is determined by the engraved scales on the lens' barrel, without even caring what the CoC is. The engraings are true for and only for, prints that are made according to the applicable assumptions. Canon assumes a print size, viewing dsitance and CoC. These assumptions do nat have to be made. Or that the DoF preview in the view finder works (what is the "print size" (maybe 1x1.5"), what is the viewing distance, and isn't the viewfinder dimmer than the print viewing area?). Or the assumptions for DoF calculators are not provided, but simply provide a CoC value for the camera/lens being used.
Assertions that DoF is dependant on pixel pitch leads me to believe not all the assumptions about DoF and DoF itself are understood.
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I merely said that assumptions were made and that the reader must understand those assumptions in order to understand Johnson's results.
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The viewing conditions and such were clearly stated, and Johnson spent some time explaining the origins of his choice of "CoC diameter of 1/1500 of the diagonal dimension of the image" so your concerns seem to have been addressed. I do not understand your implicit criticism that something important went unstated or unexplained.
I suggest that you read the three paragraphs starting with the words.
"However, there is a caveat. The type of DoF computation depends on the way photographs will be viewed!"
which includes
"There is nothing sacred about this choice of CoC. If very large prints are to be made, perhaps the CoC should be reduced; and vice versa if only 4" by 6" prints are needed one can get away with a larger CoC."
You might enjoy reading this article about the so-called [a href=\"http://en.wikipedia.org/wiki/Zeiss_formula]Zeiss formula[/url], the 1/1730 variant.
Here is an interesting quote from the first, 1997 issue of Camera Lens News a quarterly newsletter published by Carl Zeiss, interesting in that it mentions 1/1500, not 1/1730:
"According to international standards the degree of blur tolerable is defined as 1/1000th of the camera format diagonal, as the normally satisfactory value. With 35 mm format and its 43 mm diagonal only 1/1500th is deemed tolerable"
I could dig up a citation of Ansel Adams using the 1/1000 figure if you are interested.
I do think that Johnson could have made his explanation clearer at one point. He argues reasonably for CoC of about 1/1500 of the viewing distance, but then shifts to CoC of 1/1500 of the image diagonal, and the two only go together in the case of viewing distance roughly equal to print diagonal size. Johnson says that he is dealing with the case of a viewing distance of 10 to 12 inches, so seems also to be considering prints of about 8"x10" size. It is clear from the quote above that he is explicitly talking about prints of some typical size, not prints of any imaginable size. That is the universal custom, embodied for example in the DOF scales on lenses and such: makers of very larger pritns are expected to know haw to adjust for that situation.
However I would prefer to just saying that the calculations refer to the common case of
viewing distance = image diagonal size
which is often describe as "normal field of view".
One could generalize the result by noting that DOF changes from this value in proportion to the ratio (viewing distance/image diagonal size).
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I suggest that you read the three paragraphs starting with the words.
"However, there is a caveat. ..."
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Thank you, I am familiar with the article and the references you cited. However, I am not at all sue that everyone is.
"One implication of this is that the DoF scales engraved on lenses are meaningless when the lens is attached to a DSLR, since they do not take account of either sensor size or pixle pitch; and even tables like Johnson's, which do take account of sensor size, are off the mark. I'm working out a corrected one, which I hope to post later."
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Assertions that DoF is dependant on pixel pitch leads me to believe not all the assumptions about DoF and DoF itself are understood.
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Since this is a long and somewhat technical discourse, let me try to motivate the potential reader by summarizing the conclusions which I think it demonstates:
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Digital is different. Pixel pitch affects DoF in a serious way, and (except in the exceptional case when pixel pitch is exactly proportional to sensor size), the "M x A rule" and similar rules of thumb, or DoF tables based on film-camera concepts, will be inaccurate and misleading.
Both Nikon and Canon are widely rumored to be contemplating introducing large-MP full-frame "Super DSLR" models. But a "Super DSLR" such as our hypothetical 27MP "D3Z" will have considerably less DoF that existing 10-16MP full-frame DSLRs. As a practical matter, the DoF penalty for such a camera, arising from its necessarily finer pixel pitch, may be so large as to make it effectively impossible to get enough DoF for landscape photography, for example, by simply stopping down. The only recourse would seem to be using lens movements, of the type view-camera users have long been obliged to learn, in a new generation of well-corrected "Tilt & Shift" lenses. Let us hope that Nikon and Canon will attend to this task, as well.
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The mathematics of calculating DoF from the CoC are described in any standard optics text (for example, Chapter 22 of Sidey Ray's Applied Photographic Optics); the basic idea is that if the image of a scene element K units in length can be covered by a "Circle of Confusion" of diameter C on the film, it wiil not be perceived in detail, and will be in the "out-of-focus" zone; while one whose image is too large to be covered by the CoC will be detectable on the print, in the "in-focus" zone. For a camera focused on a subject u meters away, the near and far camera-to-object distances, S < u and and R > u, at which a K-sized element would just span the CoC, define the near and far limits of the "in-focus" field. The Depth of Field T is defined as T = (R-S), and for "normal" (i.e., non-closeup) distances, it can be calculated from the equation
T = (2*u*u*N*C)/(f*f)
where u is the camera-to-subject distance, f is the focal length of the lens, C is the diameter of the CoC, all in meters, and N is the relative aperture, a pure number (2.8, 4, 5.6, etc.). The CoC (value of C) is chosen in view of the intended enlargement, and distance at which the print is to be viewed; a value of 30 microns (millionths of a meter) would be typical for a 35mm negative, enlarged 8x to a 8" x 12" print, to be viewed at 10-12 inches (the 30-micron CoC would thus be enlarged to a 8 x 40 = 240-micron blur spot on the print).
To apply this apparatus to digital, we must re-express the CoC in terms of pixels, not microns, as explained in my previous post.
But let's first apply it to FILM cameras, which have no pixels, to the calculate the DoF's of full-frame vs APS. For concreteness, suppose we're using a full-frame 35mm SLR to photograph an object 10 meters away, using a 100mm lens (f = .1), using a CoC of 40 microns, at an arbitrary aperture N. If we work out the arithmetic (keeping track of all the zeroes) we find that T = .3N, implying a DoF of 1.2 meters at f/4, or 2.4m at f/8, etc., which is in the ballpark with the DoF scales engraved on a typical 100mm SLR lens.
Now suppose we take the same shot from the same vantage point with an an "APS" (16 x 24mm frame) body, using a wider lens (because of the "crop factor") to get the same FoV; the proper lens to use is one of focal length f' = (2/3)f = 68mm. Moreover, because the APS negative is only 2/3 as large, it will have to be enlarged more to yield the same final print size, by a factor of 3/2; hence, in order to end up with the same-sized blur spot on the print, we must take a smaller CoC, C' = (2/3)C. All other factors in the equation are the same, so the DoF for the APS shot, T', is
T' = [2*u*u*N*C']/f'*f' = [2*u*u*N*(2/3)C]/[(2/3)f*(2/3)*f] = (3/2)T = (3/2)*.3N
Thus, at the same aperture, the DoF in the APS image is 3/2 as large; to get the same DOF we would have to open up the aperture to N' = (2/3)N. Or, to put it the other way around, the DoF for a full-frame camera, using a longer lens to give the same FoV, is only 2/3 as deep, compared to the APS camera; and to get the same DoF we would have to stop down the full-frame's longer lens by the "crop factor" of 3/2: if the APS image (using the 68mm lens) were shot at f/4, we would have to stop down the 100mm lens on the full-frame camera to f/6.
This relationship, and the rule "stop down by the crop factor" is a general one, which does not depende on the numerical specifics of our example. This type of ananlysis is the basis for similar "rules", such as Alexander's "CoC =Diag/1500" rule, and Wrotniak's "M x A" rule.
But now let's apply this apparatus to digital, taking account of pixel pitch, specificaly to the issue of DoF on APS-C versus full-frame DSLRs, and using the D2X (an APS-C, 12MP camera with a pixel pitch of 5.5 microns) as a baseline.
The DoF of a 100mm lens focused at 10m, using a CoC = 30 microns, is T = .3N, as shown above (irrespective of the sensor size); so for the D2X, the relevant "CoC" measured in PIXELS--call it the DCoC--is 30/5.5 = 5.5 pixels in diameter. A 300-ppi print has about 12 pixels/mm, so the resulting blur spot would be about 420 microns in diameter, somewhat larger than the film-camera example used above; but this calculation ignores the effects of demosiacing & sharpening, and is close enough for illustrative purposes (suppose we're viewing all the digital prints at a somewhat greater viewing distance.) Thus the DoF for a D2X shot taken with a 100mm lens, focussed on an object 10 meters distant, is also given by T = .3N
Now let's compare the D2X image with one taken by a full-frame DSLR, in which the pixel pitch has been increased (by a factor of 3/2) to make the sensor large enough to fill the full 24 x 36mm frame. The pixel count is the same, so both cameras will yield the same (300-ppi) print size. To get the same-sized blur spot on the print, we must choose a DCoC 5.5 pixels in diameter, which for these larger pixels reqiuires a CoC of C' = (3/2)C. To get the same FoV, because of the "crop factor", we would use a lens of f' = (3/2)f = 150mm; so plugging these into the equation
T' = [2*u*u*N*C']/[f'*f'] = [2*u*u*N*(3/2)C]/[(3/2)f*(3/2)*f] = (2/3)T = (2/3)*.3N
This follows "stop down by the crop factor" rule, as did the film-camera example above.
But now suppose we use a full-frame camera--the "D3Z"--which uses the same pixel pitch as the D2X. Its 27MP image will yield a consdierably larger, 300-ppi print; but suppose we still view it at the same distance, using the same blur-spot size to define the in-focus field. To get the same blur-spot on the print, we must use the same CoC, C' = C = 30 microns, which is still 5.5 pixels in diameter; so, taking account of the longer lens, we have:
T" = [2*u*u*N*C']/[f'*f'] = [2*u*u*N*C]/[(3/2)f*(3/2)*f] = (4/9)T = (4/9)*.3N
There is now less than half the DoF; for the full-frame camera to get the same DoF as in the D2X shot, we must now stop down by the SQUARE of the "crop factor", in this case by (9/4). If the D2X shot were taken at f/4, we would have to stop the D3Z's 150mm lens to f/9--which is approaching the limit (f/11, according to Thom) at which D2X users have found that diffraction-induced blurring effects increase objectioanlly (basically because the diameter of the Airey disk exceeds the 2-pixel Nyquist limit).
There are a couple of morals to this story.
Digital is different. Pixel pitch affects DoF in a serious way, and (except in the exceptional case when pixel pitch is exactly proportional to sensor size), the "stop down by the crop factor" and similar rules of thumb, or DoF tables based on film-camera concepts, will be inaccurate and misleading.
Both Nikon and Canon are widely rumored to be contemplating introducing large-MP full-frame "Super DSLR" models. But a "Super DSLR" such as our hypothetical 27MP "D3Z" will have considerably less DoF that existing 10-16MP full-frame DSLRs. As a practical matter, the DoF penalty for such a camera, arising from its necessarily finer pixel pitch, may be so large as to make it effectively impossible to get enough DoF for landscape photography, for example, by simply stopping down. The only recourse would seem to be using lens movements, of the type view-camera users have long been obliged to learn, in a new generation of well-corrected "Tilt & Shift" lenses. Let us hope that Nikon and Canon will attend to this task, as well.
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Assertions that DoF is dependant on pixel pitch leads me to believe not all the assumptions about DoF and DoF itself are understood.
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I think those assertions about pixel pitch refer to the case of pushing enlargement and viewing distance to the resolution limits of the sensor. In other words, the minimum DOF that one could perceive with the greatest amount of enlargement and scrutiny. Quite different conditions that the ones usually discussed of viewing with "normal viewing angle" or viewing distance equal to image diagonal.
By "viewing angle", I mean the angle subtended by the longest dimension of the image (its diagonal) from the viewer's eye. Roughly this is the ratio of image diagonal to viewing distance. [To be precise, (image diagonal)/(viewing distance) = 2 * tan (1/2 viewing angle).]
It is often said that about 50º is the viewing angle that best fits the way the human eye perceives a scene, though peripheral vision is far wider and the most detailed image as seen by the fovea is far narrower. 50º gives viewing distance = 107% of image diagonal, close enough to equal.
Every calculation that gives a specific DOF value applies only to one choice of viewing angle; that is, to one particular ratio of viewing distance to image size. So long as the choice of viewing angle is stated, I have no problem: no _unnecessary_ assumptions are being made, only the _necessary_ choice of a viewing angle.
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One implication of this is that the DoF scales engraved on lenses are meaningless when the lens is attached to a DSLR, One implication of this is that the DoF scales engraved on lenses are meaningless when the lens is attached to a DSLR, since they do not take account of either sensor size or pixle pitch; and even tables like Johnson's, which do take account of sensor size, are off the mark. I'm working out a corrected one, which I hope to post later.
; ... .
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I agree the DoF scales engraved on lenses are meaningless without the assumptions, but not for the reason you give (they do not take account of either sensor size or pixle pitch).
Let's just agree to disagree. I doubt any amount or quality of agrument will change anybody's opinion.
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Let's just agree to disagree. I doubt any amount or quality of agrument will change anybody's opinion.
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Your opinion, anyway. Can't you do elemetary math? Consider a Community College.
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Your opinion, anyway. Can't you do elemetary math? Consider a Community College.
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I was hoping we could just disagree without having to get nasty. Yes, it is my opinion, but I could provide several references from knowledgible folks.
I can do (and have done) math. I could provide transcripts to prove I could at least once, but I won't. I have been to college, just not a community college.
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I was hoping we could just disagree without having to get nasty. Yes, it is my opinion, but I could provide several references from knowledgible folks.
OK, sorry--but the attack on "quality argument" is an attack on the foundations of our civilization, at least since the Enlightenment (and before the Bush42 administration).
You may be right, that mathematical arguments won't convince everyone; so what about some empirical evidence? I took some test shots to try to demonstrate the effect of pixel pitch, using the same lens on my D2X (pp = 5.5 microns) and D70 (pp = 7.9 microns). I thought I could see it pretty clearly; but the shots themselves weren't very good, taken outdoors, in a blustery wind with ever-changing lighting conditions, due to scattered clouds scudding acroos the sky; so I didn't post them.
Why doesn't someone run some more careful tests under controlled studio conditions?
Comparing the DoF results I derived for the "big-pixel FF" and the "D3Z", we see that the latter's DoF T' is related to the former's, T, by
T' = (2/3)T
(at a given aperture N); this is basically because the D3X's pixel pitch is 2/3 as small. The relation is a general one: if pixel pitch of camera A is R times that of camera B, then their DoFs (at any given apeture N) should be related by
T' = R*T
For the D2X vs D70 comparison, R = 5.5/7.9 = .7; another possible pairing would be with one of Canon's large-pixel DSLRs like the 1Ds (8.9 microns) or 1D Mark II (8.2 microns) with one of their small-pixel models. (Sensor size doesn't matter--just crop the image from the larger sensor to the dimensions of that of the smaller, to get the same FoV). The advantage of such same-brand comparisons is that can use exactly the same lens--preferably a longish, sharp prime, like a 300mm f/2.8) on both. The idea would be to set uo a spaced series of targets, well iluminated, and then run two series of shots from a sturdy tipod, starting at the widest aperture, and stopping down to the point where diffraction begins to dominate everything (around f/11, for the D2X).
I'd be interested is seeing the results.
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OK, sorry--but the attack on "quality argument" is an attack on the foundations of our civilization, at least since the Enlightenment (and before the Bush42 administration).
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I really had no idea I was attacking "quality argument." I thought I simply said no amount of it was going to change anybody's opinion. Instead of "anybody," I should have limited it to me. I apologize.
Nor did I realize I was bashing the "foundations of our civilization." What do you expect from a Bush Republican newculer engineer who has never been to community college? And if I had realized that, I would be very surprised that anybody actually cares.
And good luck with your "test."
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Digital is different. Pixel pitch affects DoF in a serious way ... a "Super DSLR" such as our hypothetical 27MP "D3Z" will have considerably less DoF that existing 10-16MP full-frame DSLRs. [a href=\"index.php?act=findpost&pid=98161\"][{POST_SNAPBACK}][/a]
If sensor (or film resolution) is increased as in these imagined future cameras, and one then views the resulting prints with the same "viewing angle" (e.g. still 11"x14" prints viewed from 15" away), with the resolution of both cameras high enough that sensor resolution is beyond the viewer's visual acuity, the DOF will not change; at least not according to any of the formulas you quote. This is because the threshold print CoC size for being in-focus does not change under these viewing conditions.
If instead the images are printed and viewed in a way that exposes the full resolution capabilities of the new sensor, so that the higher pixel count images are viewed at a larger viewing angle (larger print and/or closer viewing distance), then the extra "enlargement" reduces DOF in the higher pixel count image. (Here I count moving closer to an image as a kind of "enlargement" because it makes the image look larger.)
The mistake to avoid (I am not saying that gkramer made it) is to believe that prints of the same size as one has happily been making will suffer reduced DOF when viewed from the same distance due to the image recorded by the camera having somewhat higher resolution.
Instead, the need for smaller apertures to get adequate DOF comes mainly from the making of larger prints. This is just as true with film when planning to make larger prints that will get close scrutiny, such as when using higher resolution film.
This is not a specifically digital effect; it is an indirect effect of the extra enlargement opportunities offered by using a "light sensitive medium" of higher resolution.
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The mistake to avoid (I am not saying that gkramer made it) is to believe that prints of the same size as one has happily been making will suffer reduced DOF when viewed from the same distance due to the image recorded by the camera having somewhat higher resolution.
No, I don't think that's right--or maybe I'm just confused by what you mean by a "camera having somewhat higher resolution," and by the "enlargement" of a digital print.
"Enlargement": For given viewing conditions, the "native" 300-ppi size of a digital print is related to the pixel count, and not to the physical size of the sensor; it is the standard for close-viewed prints. "Enlargement" of a digital print above this size might refer to uprezzing to a larger pixel count, to produce a larger 300-ppi print, for close viewing; or to producing a larger print at lower ppi, for viewing at a greater distance, like a poster on a wall. But neither of these types of "enlargement" is relevant to the issues I was discussing, which is why I studiously avoided using the term, and worked out everything in terms of the 300-ppi "native" standard.
"Higher resolution": If this means a higher pixel count, then the "native" 300-ppi print size also increases, so I don't understand why we should be comparing "prints of the same size."
If, on the other hand, "higher resolution" means the how high a lp/mm value the sensor (& lens) can resolve, as measured from test charts like those used to test lenses & cameras in the photo magazines, then for a well designed digital camera, this is essentually determined by the Nyquist = 2 pixel limit (which is the lp/mm value at which the MTF cuts off) is thus essentially a matter of pixel pitch: finer pixel pitch = higher resolution.
If we decrease the pixel pitch on a sensor of fixed size, then obviously the total pixel count will increase, so we're back situation of the second paragraph: it makes no sense to compare "prints of the same size."
If, on the other hand, we reduce the size of the sensor as we decrease the pixel pitch, so as to keep the total pixel count constant, then we are at essentially the situation described in my post in comparing my hypothetical "big-pixel FF DSLR" with the D2X. In this case, the "higher-resolution" camera with the finer pixel pitch, the D2X, actually has MORE DoF, not less; so I guess that's not what you had in mind.
I'm not necessarily disagreeing with any of your points (which are perfectly sensible for film-camera world), just suggesting that the meanings of terms like "enlargement" and "resolution" are not self-evident in the digital world, and until they have been carefully redefined for digital, it's not clear that a perfectly sound proposition from the film-camera world is also valid, or even meaningful, for the digital-camera world.
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The mistake to avoid (I am not saying that gkramer made it) is to believe that prints of the same size as one has happily been making will suffer reduced DOF when viewed from the same distance due to the image recorded by the camera having somewhat higher resolution.
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I don't think it is a mistake to believe this. DoF is a perception of what is considered in-focus enough. It is easier to see a differences in sharper prints. A CoC just larger than the edge of an infocus CoC will not appear as much different than a CoC just smaller than in-focus when the image is "fuzzy." I think this is a perception and not due to pixel pitch. You can see this efect by "fuzzing" a digital image with a soften filter or blur in photoshop.
Got an image where the subject is slightly out of focus? (I do.) Blur the entire print and it won't be as noticible.
The over-all resolution O is:
1/O = 1/S + 1/L
where S and L are the resolution of the sensor (film or digital) and lens respectively. It seems strange to me that, BD (Before Digital) as lenses and film got sharper (higher resolution), the "laws" of DoF did not need to be changed. Why didn't DoF calculations change (to include film resolution) when I switched from Tri-X to T-Max 100? Or I got that new L lens? It seems only the arrival of digital has made some folks think something else has changed too.
How does one now compsre DoF between a digital camera and a film camera, all else being the same?
To quote a famous digital photographer/educator: "There was a query in October, 2001 on my Discussion Forum as to whether Depth of Field was calculated any differently for digital Vs. film. The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, ... "
Bob Atkins at photo.net says: "If you use the same lens on a EOS 10D and a 35mm film body and crop the 35mm image to give the same view as the digital image, the depth of field is IDENTICAL"
"Both Nikon and Canon are widely rumored to be contemplating introducing large-MP full-frame "Super DSLR" models. But a "Super DSLR" such as our hypothetical 27MP "D3Z" will have considerably less DoF that existing 10-16MP full-frame DSLRs."
Have great faith in industry and science, it seems highly unlikely that Nikon and Canon would produce super DSLR models and not notice somewhere along the line that there is little or no DoF, a new bag of lenses is required, there knowledge of DoF is all wrong and they need to rethink the whole mess or hire gkaramer. I could be wrong but that would rock even my foundation of civilization as I know it.
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I don't think it is a mistake to believe this. DoF is a perception of what is considered in-focus enough. It is easier to see a differences in sharper prints.[a href=\"index.php?act=findpost&pid=98325\"][{POST_SNAPBACK}][/a]
Howard, you missed my qualification about comparisons "with the resolution of both cameras high enough that sensor resolution is beyond the viewer's visual acuity". My viewing angle example of 11"x14" prints viewed from 15" away was intended to be within the realm where increasing resolution beyond that of the 1DsMkII would not produce a significant increase in perceived sharpness, and so would not produce a noticeable decrease in perceived DOF.
The situation you are talking about is the other one I discussed, where the prints are large enough to reveal the resolution limitations of the lower pixel count image, making in-focus parts of the image noticeable sharper in the prints from the higher resolution camera.
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or maybe I'm just confused by what you mean by a "camera having somewhat higher resolution," and by the "enlargement" of a digital print.
"Enlargement": For given viewing conditions, the "native" 300-ppi size of a digital print
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There is nothing "native" about 300PPI; it is a number that has acquired an unwarranted status as "the one and only true PPI choice for printing" And at a quick check, I see nothing in your previous posts stating that all comparisons are done for the case of 300PPI prints. Also, I do not believe that all un-cropped prints from the 1DsMkII are 16.6" by 11" as 300PPI dictates, so I do not see a good basis for an (unstated?) assumption of a fixed 300PPI. This is like assuming that anytime one changes to a higher resolution film, one starts making all ones prints proportionately larger.
But apparently you are assuming comparisons at equal PPI, which is the second case I was talking about, of making larger prints from the camera with more pixels.
I use "enlargement" in the well established photographic meaning of the ratio of the size of the print (or projected image) to the size of the image recorded in the focal plane of the camera. So regardless of film or sensor resolution, prints of the same degree of enlargement from the same format are the same size.
But rather than get into discussion of jargon, let me summarize this way:
1. If one compares prints of equal size, the increased resolution will not increase the OOF effects, and will have no effect on DOF (except a slight effect when the prints are so large that the ones from the camera with a mere 12 or 16MP are noticeably un-sharp everywhere.)
2. If one compares prints made at equal PPI, the larger prints from the camera with more pixels will have less DOF when viewed from the same distance, as larger prints generally do.
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Bob Atkins at photo.net says: "If you use the same lens on a EOS 10D and a 35mm film body and crop the 35mm image to give the same view as the digital image, the depth of field is IDENTICAL"
Howie,
I think Bob Atkins would have made this statement in the context that both the 10D and 35mm film have approximately the same resolution. He really shouldn't have used the word identical. He probably meant identical with respect to most practical purposes, taking an average quality of film, and ignoring pixel-peeping concerns.
Consider the following example. I use the highest resolution film I can find with my sharpest 35mm lens (say a 50mm lens with a Photodo rating of 4.6, and T-Max 100 B&W film, or Royal Gold 25 color negative). I compare equal size prints from a 4x5" camera using the same film but stopping down 4 stops with the equivalent lens (same vertical FoV) to get the same DoF equivalence. The CoC on the 4x5 negative is 4x larger. However, for the same size prints, the 4x5" film requires 4x less enlargement. I have DoF equivalence on all prints of the same size viewed from the same distance. Right?
However, supposing I'm not too happy with the slow shutter speeds I need to use with the 4x5 format and decide to use a fast, coarse grained ISO 1600 film with the 4x5 so I can freeze a moving subject, but I still use T-Max 100 with the 35mm format. I now find I can use the same shutter speeds with both cameras, in the same lighting conditions.
Have I still got DoF equivalence? I think not. The part of the 4x5 image that's in focus will now be less sharp than it was before, due to the coarse grain obscuring fine detail. At a certain degree of print enlargement, the 4x5 will be perceived as having more DoF than the 35mm format, and probably the over all resolution advantage of the 4x5 format, in large prints, will be lost.
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There is nothing "native" about 300PPI; it is a number that has acquired an unwarranted status as "the one and only true PPI choice for printing" ...I do not see a good basis for an (unstated?) assumption of a fixed 300PPI. This is like assuming that anytime one changes to a higher resolution film, one starts making all ones prints proportionately larger.
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I think this as a thoroughly inaccurate and misleading statement: the 300-ppi standard has become the accepted standard for digital prints viewed a close viewing distances (10-12")--just as a 30-micron (give or take a few microns) CoC has become the widely accepted standard for 35mm photography, based on an 8x enlargement factor (to an 8 x 12" print) and the implied 240-micron blur circle on the print, for close-viewing distances of 10-12". That (give or take a few microns) is essentially the standard on manufacturers engrave DoF scales on their 35mm lenses; and I think the 300-ppi standard has acquired essentially the same status in the digital world. By suggesting otherwise, I think you're just trying to confuse the issue; you could equally plausibly (or implausibly) attack the C0C = 30 micron standard for 35mm photography--from which I presume you would conclude that manufacturers shouldn't engrave DoF scales on their lenses.
I think that would be a dumb conclusion. Certainly the 300-ppi standard is not appropriate for producing poster-sized prints to be viewed on a wall--just as one might choose a diferent CoC for film enlargements intended for viewing on a wall. But it's easy to scale up the relevant factors, one we understand the basis on which the standard is based. A well-chosen, fixed standard (CoC = 30 microns, 300ppi), and DoF scales (or tables) based on it are very useful for the photographer who understands them, saving him the hassle of having to recalculate everything from scratch whenever he want to make a different-sized print. I think your implicit claim that "there's no such thing as a standard for digital printing" is misleading at best, if not outright obscurantism.
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...at a quick check, I see nothing in your previous posts stating that all comparisons are done for the case of 300PPI prints. Also, I do not... see a good basis for an (unstated?) assumption of a fixed 300PPI...[a href=\"index.php?act=findpost&pid=98384\"][{POST_SNAPBACK}][/a]
Do your homework.
It's possible that I screwed up the math, or somehow set the problem up in the wrong way; I which case I would be grateful to be shown the error of my ways. But as your "quick check" remark suggests, you haven't made any real effort to understand the argument(s), and frankly, I don't think you grasp the issues.
I use "enlargement" in the well established photographic meaning of the ratio of the size of the print (or projected image) to the size of the image recorded in the focal plane of the camera. So regardless of film or sensor resolution, prints of the same degree of enlargement from the same format are the same size.
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I think this remark proves my point: sensor size (in physical dimensions) is utterly irrelevant; as I pointed out in my first post, a 6MP, 2000 x 3000-pixel image file is an 6MP, 2000 x 3000-pixel image file, "irrespective of whether it was captured by a 24mm x 36mm full-frame sensor, a 16mm x 24mm APS-C sensor, or a tiny, quarter-inch sensor from a pocket point-n-shoot." Stripped of its EXIF data and loaded into PhotoShop for editing, resizing, and printing, neither Photoshop nor the printer would have any idea of whether it came from a full-frame DSPR, or a tiny pocket point-n-shoot, or whatever; nor would it matter. Any sensible discussion of "enlargement" (ie, how big a print we want to make from the file) in the digital world must take account of that reality. Obviously you haven't grasped the point.
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I think Bob Atkins would have made this statement in the context that both the 10D and 35mm film have approximately the same resolution. He really shouldn't have used the word identical. He probably meant identical with respect to most practical purposes, taking an average quality of film, and ignoring pixel-peeping concerns.
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Well, he did use the word identical.
You seem to dismiss Mr. Atkins statement with he didn't mean it, or should have said something different.
You did not address: "There was a query in October, 2001 on my Discussion Forum as to whether Depth of Field was calculated any differently for digital Vs. film. The answer is, no. There is no difference whosesoever. DOF doesn't care about the recording media type or size, ... "
Is this author also mistaken or over speaking? What did he really mean to say?
I didn't know there were homework assignments for this forum. Maybe you should check your own work, rather than demand others do it for you. You seem to be alone in your thinking, but that doesn't mean you are necessarily wrong. But for such a simple and well travelled topic, probably.
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you could equally plausibly (or implausibly) attack the C0C = 30 micron standard for 35mm photography--from which I presume you would conclude that manufacturers shouldn't engrave DoF scales on their lenses.
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On the contrary, the COC=30 micron standard for 35m format supports my point: it depends on the format size and not on film or sensor resolution.
The DOF scales on lenses are based on making prints of a certain reference size (5"x7"?), and viewing them from a certain standard distance (10"?) and calling something in-focus if the resulting CoC on the print in no larger than about 1/1500 of the viewing distance, on the basis that this is about the limits of what the human eye can resolve. The print CoC threshold is thus about 150 microns, and for 35mm format, the degree of enlargment needed to get a 5"x7" print is about 5x, so the CoC for DOF scales is about 1/5th of 150 microns, or 30 microns.
Note:
1) These DOF scales do depend of format size, since this determines the degree of enlargement needed to get the standard sized print. The DOF scale on medium format cameras are typically computed using a value larger than 30 microns: something like 1/1000th of the format diagonal length is common.
2) These DOF scale do not depend on the resolution of the film or sensor used.
3) These DOF scales are based on comparing prints of the same size regardless of film or sensor resolution whereas your 300PPI comparison involves comparing prints of different sizes from cameras with different pixel counts.
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On the contrary, the COC=30 micron standard for 35m format supports my point: it depends on the format size and not on film or sensor resolution.
The DOF scales on lenses are based on making prints of a certain reference size (5"x7"?), and viewing them from a certain standard distance (10"?) and calling something in-focus if the resulting CoC on the print in no larger than about 1/1500 of the viewing distance, on the basis that this is about the limits of what the human eye can resolve. The print CoC threshold is thus about 150 microns, and for 35mm format, the degree of enlargment needed to get a 5"x7" print is about 5x, so the CoC for DOF scales is about 1/5th of 150 microns, or 30 microns.
Note:
1) These DOF scales do depend of format size, since this determines the degree of enlargement needed to get the standard sized print. The DOF scale on medium format cameras are typically computed using a value larger than 30 microns: something like 1/1000th of the format diagonal length is common.
2) These DOF scale do not depend on the resolution of the film or sensor used.
3) These DOF scales are based on comparing prints of the same size regardless of film or sensor resolution whereas your 300PPI comparison involves comparing prints of different sizes from cameras with different pixel counts.
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1) The standards are format dependant. DoF is not. One does not have to adopt any standard one does not want to. I choose not to adopt a standard, but to allow myself to make any size print from any crop of any format. It is easy math (add, subtract, multiply and divide). Just use the same equations used to calculate the standard and use your own design.
2) I agree with this one.
3) Again, the scales do depend on comparing the same size prints under the same viewing conditions. The factors used in those standards do not have to the same a photographer actually uses to design a print. He can use his own print. If I want a certain DoF of an 9x 11 print under certain conditions, from a cropped 35mm negative, I can do that. If I want a "standard" print, I can make that too. I don't need any standards, DoF scales, DoF tables or on-line calculators. I don't even have to know the standards if I don't misuse and tool based on them.
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1) The standards are format dependant. DoF is not. One does not have to adopt any standard one does not want to. ...
2) I agree with this one.
3) Again, the scales do depend on comparing the same size prints under the same viewing conditions. ...
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Indeed! I think that Michael made the basic point in an essay on DOF somewhere on this site. To paraphrase
1) DOF scales on lenses, DOF tables and most DOF calculators give an "indication" of DOF under certain stated "reference" conditions, something like printing the entire image recorded by the camera and viewing the print from a distance a bit greater than the diagonal length of that image; what I will call "reference viewing distance".
2) From this guidance, a photographer planning for different viewing conditions can make adjustments. For example, if one plans on making a 16"x20" print that is likely to be viewed from as close as 15", the DOF will be only about half what the DOF scales indicate, and one would need to stop down to twice the f-stop suggested by such scales. Likewise cropping by 2x (using only half the height and width of the full image) and then viewing from "reference viewing distance" would give about half of the indicated DOF.
After all, it would be messy to mark lenses with DOF scales that cover all the possible viewing conditions for prints!
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Indeed! I think that Michael made the basic point in an essay on DOF somewhere on this site. To paraphrase
1) DOF scales on lenses, DOF tables and most DOF calculators give an "indication" of DOF under certain stated "reference" conditions, something like printing the entire image recorded by the camera and viewing the print from a distance a bit greater than the diagonal length of that image; what I will call "reference viewing distance".
2) From this guidance, a photographer planning for different viewing conditions can make adjustments. For example, if one plans on making a 16"x20" print that is likely to be viewed from as close as 15", the DOF will be only about half what the DOF scales indicate, and one would need to stop down to twice the f-stop suggested by such scales. Likewise cropping by 2x (using only half the height and width of the full image) and then viewing from "reference viewing distance" would give about half of the indicated DOF.
After all, it would be messy to mark lenses with DOF scales that cover all the possible viewing conditions for prints!
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I'm sorry. I thought you were once argueing DoF was format dependant. I must have been mistaking you for someone else, like Ray.
Yes, the photographer can use the standards as guidance. But why bother to use the standard and adjust it, when all you need do is smiply calculate DoF for your own use? Or the photographer can dumb down the whole thing and use tables, calcuators or engravings just like they were correct for his 20x24.
Yes, it would be messy to put all possible DoFs on a lens barrel. So don't put any, which seems to be the current trend. I say better none than misusing one.
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Is this author also mistaken or over speaking? What did he really mean to say?
I didn't know there were homework assignments for this forum. Maybe you should check your own work, rather than demand others do it for you. You seem to be alone in your thinking, but that doesn't mean you are necessarily wrong. But for such a simple and well travelled topic, probably.
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I think Bob Atkins might be at least partially wrong on this issue, but I understand that anyone who has not made an attempt to work out this issue for himself, would probably prefer to take Bob Atkins word for it rather than mine.
That doesn't worry me because as a photographer, ultimately, it's the practical significance of such theories that matter. If my views differ from those of more recognised authorities, it's because I've done my homework in the form of practical experiments. If I'm not sure, for example, if the sharpness and detail of an image will be compromised by using f16 or f22 with a particular lens and camera, I'll do tests on real world images, as well as line charts. If I find, for example, that there's no significance resolution difference between f8 and f16 with one lens, and no significant difference between f22 and f11 with another lens, then no amount of theoretical pronouncements from recognised authorities, whether they have a PhD or not, will dissuade me from my view. All I could say in their defense is, they must have been referring to a different set of conditions.
It's quite clear to me that system resolution affects DoF outcomes at a practical level. I carried out the following experiment recently after purchasing a Canon 50/1.4 lens. I took a series of shots at both f8 and f1.4, focussing at various distances for each pair of shots. On examining the images, I discovered that the OoF parts of some of the f8 shots were just as sharp as the in-focus parts of the f1.4 shots. The difference in sharpness in these f8 shots, between OoF parts and the in-focus parts was sufficient to create the impression of a slightly shallow DoF.
Now it's clear to me that, whilst the F1.4 shots had significantly shallower DoF than the f8 shots, the DoF would not have been as shallow as the DoF calculators and formulas would imply because such formulas do not take into account lens quality.
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I think Bob Atkins might be at least partially wrong on this issue, but I understand that anyone who has not made an attempt to work out this issue for himself, would probably prefer to take Bob Atkins word for it rather than mine.
That doesn't worry me because as a photographer, ultimately, it's the practical significance of such theories that matter. If my views differ from those of more recognised authorities, it's because I've done my homework in the form of practical experiments. If I'm not sure, for example, if the sharpness and detail of an image will be compromised by using f16 or f22 with a particular lens and camera, I'll do tests on real world images, as well as line charts. If I find, for example, that there's no significance resolution difference between f8 and f16 with one lens, and no significant difference between f22 and f11 with another lens, then no amount of theoretical pronouncements from recognised authorities, whether they have a PhD or not, will dissuade me from my view. All I could say in their defense is, they must have been referring to a different set of conditions.
It's quite clear to me that system resolution affects DoF outcomes at a practical level. I carried out the following experiment recently after purchasing a Canon 50/1.4 lens. I took a series of shots at both f8 and f1.4, focussing at various distances for each pair of shots. On examining the images, I discovered that the OoF parts of some of the f8 shots were just as sharp as the in-focus parts of the f1.4 shots. The difference in sharpness in these f8 shots, between OoF parts and the in-focus parts was sufficient to create the impression of a slightly shallow DoF.
Now it's clear to me that, whilst the F1.4 shots had significantly shallower DoF than the f8 shots, the DoF would not have been as shallow as the DoF calculators and formulas would imply because such formulas do not take into account lens quality.
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So Ray, just where do you stand on DoF now. I am having trouble figuring out your position.
Is it that no theory is any good and only your "tests" will tell? Do you record test results for field use, or do you still go by the "if I think I need more , I stop down one more"? I am impressed if you can remember that such and such a lens at f/whatever and focused at x feet gives a DoF on an A3 print viewed at y inches of whatever feet. Actually, I really doubt both. You seem to be constantly changing positions to keep your nose pointed upstream.
You seem to put a lot of faith in Albert Einstein, but relativity was just his theory. He was unable to put it to much of a test (except e=mc^2 peraps). Al just wasn't able to accelerate mass to nearly the speed of light to check some things.
Do you have ideas or are you simply contrary?
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So Ray, just where do you stand on DoF now. I am having trouble figuring out your position.
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Pretty much the same as it has been for the last few years, Howie. If a print looks sharp all over, it has great DoF. The fuzzier the parts which are not in focus are, the shallower the DoF. I don't take slide rules, DoF calculators, tape measures or laser distance finders with me when I go out shooting. I rely upon experienced-based judgement for selection of aperture, shutter speed and point of focus. Sometimes I get it wrong.
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Pretty much the same as it has been for the last few years, Howie. If a print looks sharp all over, it has great DoF. The fuzzier the parts which are not in focus are, the shallower the DoF. I don't take slide rules, DoF calculators, tape measures or laser distance finders with me when I go out shooting. I rely upon experienced-based judgement for selection of aperture, shutter speed and point of focus. Sometimes I get it wrong.
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I agree DoF does depend to some degree on overall resolution. Fuzzier prints make it harder to see where the edges of DoF are. Afterall, DoF is not binary - in-focus or not in-focus. CoC gradually gets larger as the DoF limit is reached. A viewer does not see an abrupt change in focus.
And I also agree that judgement and experience with particular equipment plays a role.
Would you agree that simple DoF theory (limited to four factors - focal length, f/stop, focus distance and degree of enlargement) is at least a good place to start looking for the boundaries of DoF (near and far in-focus limits)?
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Would you agree that simple DoF theory (limited to four factors - focal length, f/stop, focus distance and degree of enlargement) is at least a good place to start looking for the boundaries of DoF (near and far in-focus limits)?
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Yes, I would, which is why I wouldn't object too much to Bob Atkins' use of the word 'identical' in comparing the methods of calculating DoF using a 10D and 35mm film. The formula is the same, the lenses are the same, the CoC chosen might be a bit smaller for the 10D, but over all system resolution is similar.
However, I think it would be true to say that the greater the disparity between the resolving power of any two systems compared, the greater the inaccuracy of any formula will be that excludes resolution considerations, whether it be the resolving power of the lenses used, or the resolving power of the film or sensor.
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Yes, I would, which is why I wouldn't object too much to Bob Atkins' use of the word 'identical' in comparing the methods of calculating DoF using a 10D and 35mm film. The formula is the same, the lenses are the same, the CoC chosen might be a bit smaller for the 10D, but over all system resolution is similar.
However, I think it would be true to say that the greater the disparity between the resolving power of any two systems compared, the greater the inaccuracy of any formula will be that excludes resolution considerations, whether it be the resolving power of the lenses used, or the resolving power of the film or sensor.
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OK. I think Bob Atkins is more right than wrong, because system resolution changes are very small compared to the calculated DoF.
The overall resolution O of a camera ans sensor is:
1/O = 1/S + 1/L
where S and L are the resolution of the sensor and lens respectively. But when you make a print, which is usually the ultimate goal, the resolution of the print P must be added. Then:
1/O = 1/S + 1/L + 1/P
So no matter how good the lens and/or sesnsor get, the overall resolution O will be no better than the print P. And print resolution isn't all that good.
The resolution of the eye (the link between print and brain) should also be added on. We have already taken it into account with CoC on the print.
If you are making comparisons of two files on your monitor, don't forget they can be mo larger than the final print, and viewed no closer. And the monitor will likely be brighter (and easier to see) than a print.
From a Reichmann tutorial on DoF and focal length:
"There are those that will no doubt find fault with either my fundamental assertion or with this test. Yes, I know that there are some flaws with both, among them that I am not taking into account diffraction and other second order effects. But, from a practical point of view, what really counts to photographers working in the real world is what ends up on a print in front of them. The fine points of optical theory are one thing, prints hanging on the wall are another. My orientation as a photographer and as a teacher is toward the pragmatic." [Emphasis added]
It appears the tutorial agrees that lens and sensor resolution effects may be second order. I have found that to be true when actually making prints instead of computerized "tests."
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If you are making comparisons of two files on your monitor, don't forget they can be mo larger than the final print, and viewed no closer. And the monitor will likely be brighter (and easier to see) than a print.
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I don't forget this and often click the 'print size' button at the top of the Photoshop page. All detail, including noise in the shadows, tends to be revealed in my prints, although it has to be said that such noise (as well as shadow detail) is more apparent when the print is viewed in good lighting conditions.
The problem with DoF on the print is that the perception of it can change, not only with the viewing distance but also with the size of background objects in the composition. Consequently, if we take two shots with different lenses, keeping the main subject the same size on the sensor or film, the objects in the background in the shot taken with the wider angle lens, will appear to be further away. The background will be more extensive. The same background objects common to both prints will be smaller in the shot taken with the wider angle lens, and as a consequence they will appear sharper.
In other words, if we accept that the DoF formulas for prints only apply accurately when consistent with viewing distances of, say the diagonal of the print, whatever the size of the print, then it follows that a wide angle lens that reduces the magnification of the background will have a similar effect to viewing the print from another 'less wide angle' lens from a greater distance (main subject occupying the same space on the sensor in both cases). I wish I could think of a less convoluted way of expressing that .
I'll try again. The assertion that all lenses produce the same DoF at the same f stop, provided the main subject is the same size on the sensor is not correct when viewing same size prints from the same distance.
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I don't forget this and often click the 'print size' button at the top of the Photoshop page. All detail, including noise in the shadows, tends to be revealed in my prints, although it has to be said that such noise (as well as shadow detail) is more apparent when the print is viewed in good lighting conditions.
The problem with DoF on the print is that the perception of it can change, not only with the viewing distance but also with the size of background objects in the composition. Consequently, if we take two shots with different lenses, keeping the main subject the same size on the sensor or film, the objects in the background in the shot taken with the wider angle lens, will appear to be further away. The background will be more extensive. The same background objects common to both prints will be smaller in the shot taken with the wider angle lens, and as a consequence they will appear sharper.
In other words, if we accept that the DoF formulas for prints only apply accurately when consistent with viewing distances of, say the diagonal of the print, whatever the size of the print, then it follows that a wide angle lens that reduces the magnification of the background will have a similar effect to viewing the print from another 'less wide angle' lens from a greater distance (main subject occupying the same space on the sensor in both cases). I wish I could think of a less convoluted way of expressing that .
I'll try again. The assertion that all lenses produce the same DoF at the same f stop, provided the main subject is the same size on the sensor is not correct when viewing same size prints from the same distance.
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Maybe you need to take this up with Michael.
I made some assumptions and did some calcualtions. With a focal fength of 50, a focus distance of 100, f/stop of 8 and a CoC of 0.1, I get DoF of about 7 feet, 4 feet in front and 3 feet behind the subject. If I double the focal length to 100 and double the focus distance to 200 (I think the focused image will be the same size), I still get about 7 feet of DoF, 4 in front and 3 behind. Just as expected.
Why should I care that an object not in focus is a different size on my two negatives? The CoC is a point 4 feet in front of (or 3 feet behind) the critically focused subject that appears as a fuzzy disc 0.1 in diameter on both negatives. Both negatives will be enlarged the same so all fuzzy discs (regardless of size, i.e., distance from the critically focused subject) will be enlarged exactly the same to make two equal sized prints. Two images taken with two different focal length lenses at two different focus distances are just that - two different images. The DoF is the same but the images are different. I think the difference is called perspective - nothing to do with DoF.
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Why should I care that an object not in focus is a different size on my two negatives?
You might as well ask, 'Why should I care if an object that is in focus is the same size on the negative or sensor?'
You could also ask, 'Why should I care if DoF appears to increase as I step back from the print?'
As I recall, Howie, you have maintained a position that choice of CoC is dependent on the viewer's perspective of the final print. If you intend making a postcard size print, you can afford to use a larger CoC. If you intend making a large print that can only be viewed from a significant distance (because perhaps there are obstacles preventing the viewer moving closer), then you can also afford to use a larger CoC. If you intend making a large print that is likely to be viewed close up, then you have to either choose a CoC which is much smaller or take the risk of people like me making comments like, 'Such a pity the foreground is out of focus'.
Two images taken with two different focal length lenses at two different focus distances are just that - two different images. The DoF is the same but the images are different. I think the difference is called perspective - nothing to do with DoF.
Well, you can't have it both ways, Howie. If you agree that the perception of DoF on the print changes according to the viewer's perspective (distance from the print), then it seems rather illogical to maintain that changing the perspective of the camera does not change DoF.
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Well, you can't have it both ways, Howie. If you agree that the perception of DoF on the print changes according to the viewer's perspective (distance from the print), then it seems rather illogical to maintain that changing the perspective of the camera does not change DoF.
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Further to this interesting dilemma, there is a solution proposed by Charles Sydney Johnson in his recent article, Lens Equivalents. I quote the relevant passage below.
However, there is a caveat. The type of DoF computation depends on the way photographs will be viewed! In principle a photograph should always be viewed from its proper perspective point. That is to say, the angle subtended by the photograph at the eye should be the same as the field of view of the lens used to take the photograph. Therefore, photographs taken with a wide angle lens should be held close to the eye so as to fill much of the field of view while telephoto photographs should be held farther away. It is sometimes forgotten that perspective in a photograph depends only on the position of the lens relative to the subject (object).
So how will the photographs be viewed? In fact, prints are usually viewed from 10” to 12” regardless of the focal length of lens used to make the photograph. When mounted prints are viewed, observers typically stand about the same distance from all prints. Observers generally don’t know what focal length lens was used, and they simply react to the apparent distortions present when a wide angle photograph is viewed from a distance greater than the perspective point. Similarly there is an apparent flattening effect when telephoto photographs are viewed too close to the eye. Also, in photographic shows the audience remains seated at the same distance from all prints and from the projection screen. Under usual viewing conditions, it is appropriate to compute the DoF with a constant CoC in the image regardless of the focal length of the lens.
The first part of this quote, as I understand it, is basically saying if you use a wide angle lens from a closer distance, then, even though the resulting prints are the same size and the subject is the same size as in another print of the same subject taken with a longer focal length lens from a greater distance, the shot taken with the wider angle lens should be viewed from a closer distance in order to maintain the perception of equal DoF that is implied by the basic DoF formulas.
Let's flesh this out a bit with a concrete example. Let's take your example of 2 shots taken with a 50mm lens and a 100mm lens, the shot with the 50mm lens being taken from half the distance to the subject so that the subject is the same size on the sensor. Let's also assume that there is some significant background detail common to both shots, say a rather OoF house some distance behind the subject.
The fundamental DoF formulas are basically saying, in both shots the actual resolution of the house is the same. If you were to enlarge the house in both images on your monitor, so both houses were the same size, you would see the same amount of detail in both houses. I know because I've tried it. (In case anyone is confused, we're using the same f stop with both lenses and have focussed on the same subject in front of the house.)
If we make equal size prints of both scenes and view both prints from the same distance, say the diagonal of the print so we can appreciate the fine detail of the subject, say Howard, then we will find that the house in the 50mm shot appears sharper. It appears sharper because it is smaller on the print. The DoF therefore appears greater in the 50mm shot.
If we move closer to the print of the 50mm shot, closer than its diagonal, the subject, Howard, will appear to be less sharp and the more distant house will appear to be more sharp. At some appropriately different viewing distance, both prints will appear to have the same DoF and we shall all be able to sleep soundly knowing that our mathematically based DoF formulas are accurate.
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You might as well ask, 'Why should I care if an object that is in focus is the same size on the negative or sensor?'
You could also ask, 'Why should I care if DoF appears to increase as I step back from the print?'
As I recall, Howie, you have maintained a position that choice of CoC is dependent on the viewer's perspective of the final print. If you intend making a postcard size print, you can afford to use a larger CoC. If you intend making a large print that can only be viewed from a significant distance (because perhaps there are obstacles preventing the viewer moving closer), then you can also afford to use a larger CoC. If you intend making a large print that is likely to be viewed close up, then you have to either choose a CoC which is much smaller or take the risk of people like me making comments like, 'Such a pity the foreground is out of focus'.
Well, you can't have it both ways, Howie. If you agree that the perception of DoF on the print changes according to the viewer's perspective (distance from the print), then it seems rather illogical to maintain that changing the perspective of the camera does not change DoF.
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'Why should I care if an object that is in focus is the same size on the negative or sensor?"
They are the same size simply because I designed them that way when choosing focal length and focus distance.
"Why should I care if DoF appears to increase as I step back from the print?"
Simply because the eye cannot see the CoC that did appear at the limit any longer and a larger CoC becomes the limit, increasing DoF. DoF does not "appear" to increase, it does.
DoF does change as viewing distance changes. Look at the assumption made in selecting CoC concerning the size disc that can be seen - viewing distance dependent.
All I can say about stuff on the negative is, once the image is made, the negative is fixed. Changing camera position does change DoF deign, but that can be be accounted for in the deign process by changing the focal length lens.
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Further to this interesting dilemma, there is a solution proposed by Charles Sydney Johnson in his recent article, Lens Equivalents. I quote the relevant passage below.
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Charles Johnson says prints should be viewed from the proper perspective distance. We know it is not true that they are. Otherwise all pages of a book would have to have images with the same perpsective. They usually don't.
Images can be viewed in a book correctly if the images are designed correctly to be the proper size and viewed from a proper distance.
Perspective is only the focus distance and focal length - thinks that affect the size relationship of things on the negative. These are both factors in DoF but not the only factors. DoF on the print is based also on f/stop and CoC (enlargement of the negative). CoC also depends on the viewer's conditions (distance being the most common).
All I can offer now is if it works for you, Ray, do it. I know what works for me, and I will continue doing it.
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'Why should I care if an object that is in focus is the same size on the negative or sensor?"
They are the same size simply because I designed them that way when choosing focal length and focus distance.
So let's get this straight, Howie. Having designed that the main subject, in focus, is a certain size on your negative, you don't really give a stuff about the size of the objects in your image which are 'not in focus'. Is that right? Out of focus, out of mind, whether it's a little out-of-focus or a lot out-of-focus. Is that your position?