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Author Topic: Newbe question 35mm vs MF and dof  (Read 44992 times)

Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #100 on: January 04, 2011, 11:44:36 pm »

OK, I'll continue to bite...

I'm familiar with that article.

That article is about resolution and diffraction errors. It's about resolving line pairs, which is what your formula is correctly used for. This is not the correct approach to use when talking about DOF because that's about a threshold at which a single point becomes seen as a disc, it's not about distinguishing between line pairs - not the same thing at all. And I didn't want to raise the subject of diffraction errors so as not to muddy the waters.

The CoCs referred to in this article are simply not the same as the ones referred to in DOF calculations. Same term, but being used in the context of resolving line pairs as opposed to DOF calculations. You entire premise is built on this wrong interpretation of correct information, you are taking certain facts out of context. Accept this and all the information which you claim is incorrect becomes magically corrected - even the Alpa document, which is totally correct if read with this in mind. Do you really think they would publish something as flawed as you claim?
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Nick Rains
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #101 on: January 04, 2011, 11:59:43 pm »

http://www.luminous-landscape.com/essays/Leica-M8-Perspective.shtml

Same authors of the resolution article you referred to, calculating DOF for a Leica M8.

"The circle of confusion is a conventional value. It depends on the size of the sensor, the size of the print and the particular vision capabilities and subjectivity of the observer."

Note that they are NOT taking the COC as derived solely from the sensor specifications.These guys know what they are talking about, far more than I do. if you won't believe me, how about believing these guys?
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Nick Rains
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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #102 on: January 05, 2011, 12:15:41 am »

I have not invented anything new here, and I have only cited well known DOF formulas.  I have not disagreed with any of the DOF formulas that you or anyone else here has linked or referenced.  Please show me where you think I presented a DOF formula that is different.

As for the CoC formula I presented, it is the same as also used by authors of a well-known article published here on Lula:

http://www.luminous-landscape.com/tutorials/resolution.shtml

It appears that another article you cited contradicts your position.  The authors of the above article state in their section on circle of confusion...

The maximum point size the human eye cannot see as a separated point in a print corresponds to a point of a particular size on the negative. This is called circle of confusion (CoC).

Strange that they would mention the need for a print when discussing what the circle of confusion means. :D

They then spend the remainder of the section on circle of confusion discussing human visual acuity, print sizes, and how viewing at 100% on screen is the equivalent of a large print. They ultimately conclude that section by pointing out that if you want to truly evaluate the performance a digital system, you need to step away from "relative" and "subjective" measurements such as the circle of confusion, and instead look at resolution.  This clearly shows the authors believe the CoC to be a subjective number, one based on human perception of a print/image.

And if CoC is a subjective number, then DOF is subjective and not absolute because the choice of CoC is what drives the DOF equation. 

So let's tally up the score thus far according to your position...

People who are wrong about DOF:
Ansel Adams
Zeiss
Alpa
Michael Reichman
Norman Koren
Bob Atkins
Efraín García
Juan Oliver
Javier Martín
Rubén Osuna
Nick Rains
everyone else in this thread.

People who are correct about DOF:
David Klepacki

Do you have anyone or any source that you can cite who actually agrees with your position?
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ErikKaffehr

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Re: Newbe question 35mm vs MF and dof
« Reply #103 on: January 05, 2011, 12:23:22 am »

Hi,

I may suggest that the question is what is acceptably sharp?

- How large do you want to print? If the picture turns out very good yo perhaps want to print it very large?
- Are you investing in expensive glass and back to make images acceptably sharp

For critical sharpness DoF is very short. With modern sensors and good lenses at large apertures what you have focused on will be sharp and not much else.

My view is that for optimal sharpness the main subject must be in focus, not just within a calculated DoF. Than we try to expand DoF by stopping down.

What I have seen at CoC of two times the pixel pitch reduces sharpness significantly. This is illustrated in the article below:
http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures

You can get away with pretty much unsharpness in print. The image below prints very well on A2:
http://echophoto.dnsalias.net/ekr/DepthMerge/20091010-CratersOfTheMoon_04.jpg

Best regards
Erik




http://www.luminous-landscape.com/essays/Leica-M8-Perspective.shtml

Same authors of the resolution article you referred to, calculating DOF for a Leica M8.

"The circle of confusion is a conventional value. It depends on the size of the sensor, the size of the print and the particular vision capabilities and subjectivity of the observer."

Note that they are NOT taking the COC as derived solely from the sensor specifications.These guys know what they are talking about, far more than I do. if you won't believe me, how about believing these guys?
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #104 on: January 05, 2011, 12:32:12 am »

Hi,

I may suggest that the question is what is acceptably sharp?

- How large do you want to print? If the picture turns out very good yo perhaps want to print it very large?
- Are you investing in expensive glass and back to make images acceptably sharp

For critical sharpness DoF is very short. With modern sensors and good lenses at large apertures what you have focused on will be sharp and not much else.

My view is that for optimal sharpness the main subject must be in focus, not just within a calculated DoF. Than we try to expand DoF by stopping down.

Best regards
Erik


Absolutely. Theoretical arguments aside, i have not used a DOF calculator for years. The S2 is incredibly unforgiving and most MFDB users find out very soon that the DOF they were used to no longer holds up.

Basically as long as the subject is critically sharp, everything else is as sharp as it can be. Beyond f8-11 the trade off between acceptable sharpness and diffraction error becomes a significant question.
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Nick Rains
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David Klepacki

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Re: Newbe question 35mm vs MF and dof
« Reply #105 on: January 05, 2011, 01:37:16 am »

Nick, first I did not realize that I may have inadvertently insulted you in my previous response.  So, I apologize if it came across that way. You are obviously an intelligent guy who knows his stuff and open minded to boot.

I understand the tradition in photography to use CoC values based on print size.  This is because the print is the ultimate goal of the photographer and the desire is to be able to manipulate the optical situation in order to meet these printing goals.  So, the natural thing is to attempt to relate the desired perceived depth in the print to the photographic capture process, and this is exactly what has been done.  This is indeed the photographer's position on DOF matters, as articulated in this thread by yourself and others here.

From the strictly scientific perspective, this use of DOF is not "correct" according to the laws of optics as I pointed out in an earlier post of this thread.  I do admit that the traditional photographic usage serves its intended purpose, but the science of optics does not make reference to printing when referring to DOF.  This disparity is the fundamental source of the debates in this thread.  From a scientific perspective, the emphasis is on having objective values for CoC due to desired needs to determine a DOF of maximum resolution for example, whereas in photography the emphasis is more or less subjective due to the specific need to correlate the perceived depth of "apparent sharpness" in the final print.

So, from a scientific perspective, objective values for CoC used to estimate the depth of field containing the maximum possible resolution for a given sensor are obtained by attempting to relate CoC to the maximum or optimal sampling rate.  Typically, such a CoC is related to the Nyquist limit associated with the sensor under the constraint of the Airy disk associated with the lens.  Ignoring the lens issues for simplicity, this would imply simple estimates for CoC such as 2*p for monochromatic sensors (and often used for Bayer sensors by assuming perfect raw conversion as a best case scenario).  In Bayer sensors, the closest adjacent pixels of the same color channel (green) are often used as a compromise estimate, which happens to be the length of the on-center diagonal distance between two green pixels.  Or, a worst-case scenario is sometimes used as the lowest sampling frequency of the sensor (red or blue channels), which amounts to a CoC of 4*p.  

As you imply, photographers may not give a hoot about this use of DOF, and are only interested in what they need to do in order to achieve a desired perceived depth at their intended print size.  However, there is some value in the scientific approach.  By using a CoC that corresponds to the maximum possible sensor resolution, you will be able to determine the maximum size print which will satisfy a particular viewing resolution, and beyond which size you may begin to notice some blur being introduced.  Or maybe you want to know how much larger you can print with a bigger sensor and get the same sharpness or perhaps be able to see more resolution in a print of a given size.  I believe these kinds of questions are always of interest to some photographers, and without using DOF and CoC values that are related to sensor resolution, you will not be able to estimate their answers.

Getting back to your S2, I would say those lenses are state of the art diffraction limited designs.  Their MTF is also the highest in the industry.  And, the raw conversion that you use is most likely of extremely high quality.  This would result in a "scientific" estimate of CoC to be 2*p for a DOF containing optimal resolution.
« Last Edit: January 05, 2011, 01:39:46 am by David Klepacki »
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #106 on: January 05, 2011, 01:50:28 am »

Good. So, as photographers, which most of us in these forums are, we can still say that the print size has an effect on the DOF.

Well thank heavens for that!
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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #107 on: January 05, 2011, 01:59:48 am »

A very articulately written statement, which can be summarized generally as:

"No, I am not wrong. But yes, I admit that in general the world of photography takes a position contradictory to what I have been saying."

Thank you for the kind and civil discussion David.
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Bart_van_der_Wolf

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Re: Newbe question 35mm vs MF and dof
« Reply #108 on: January 05, 2011, 07:23:52 am »

A very articulately written statement, which can be summarized generally as:

"No, I am not wrong. But yes, I admit that in general the world of photography takes a position contradictory to what I have been saying."

Thank you for the kind and civil discussion David.

Hi Sheldon,

I agree with that conclusion, and I add thanks for a daily dose of entertainment. Also thanks to the contributors to this thread who are knowledgeable enough to resist derailing tactics commonly used by trolls. Thank goodness none of those are to be found around here ... ;)

And now for another piece of relevant information; One of the most authoritative sources on optics I've seen on the internet, Paul van Walree, suggests a simple formula for the calculation of the COC "Sharpness criterion" (as always as a function of output magnification and viewing distance) in the PDF manual with his VWDOF (Windows) application on page 3 and 4:

A better approach is to determine the required COC value C yourself:
C = V / (1000 * Q * Mp)

Where:
C = Circle of Confusion in millimetres to be used in DOF formulae,
V = Viewing distance in millimetres,
Q = Quality factor, Q=1 for Conventional, Q=2 for Demanding, or Q=3 for Critical sharpness
Mp = Output magnification factor, Output size divided by Sensor array size

This very simple formula holds very well in the daily practice of planning different shooting/presentation scenarios.

Of course for a more tailored answer to the OPs question, we therefore need more input, but failing that let's assume the following scenario (see attachment for a VWDOF output).
Leica S2 with a 70mm lens @ f/2.5, focused at 10 metres distance, versus
Nikon D3x with a 50mm lens @ f/1.6 focused at 10 metres distance.

If their COCs (assuming critical sharpness is required, Q=3) are scaled in proportion (!) to their sensor array diagonals, then they will both have about the same DOF at 10 metres distance, close to 1.27 metres in total. The exact DOF will vary with output magnification, but it will scale in proportion for both camera/lens scenarios, because the COCs have been scaled in anticipation of their required magnification to same size output. However, there are still some differences, e.g. the amount of blur at other distances (e.g. at infinity, or at 1 metre behind the plane of focus). Unfortunately no simple factor can explain it all, but a good DOF calculator is always handy for these sorts of questions (provided one uses the right input).

Of course this also doesn't tell us anything about differences in optical quality nor in handling ease of such different platforms.

Cheers,
Bart
« Last Edit: January 05, 2011, 07:27:47 am by BartvanderWolf »
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #109 on: January 05, 2011, 07:59:03 am »

Hang on, David is actually right, not generally, but specifically. It suddenly dawned on me what he was on about, and I always had a niggle at the back of my mind that I was missing something.

Look, it's really late here, I'm off to bed but I'll tell you what Davids arguments really mean tomorrow...I don't mean to tease but I need to marshall my thoughts a bit more.
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ErikKaffehr

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Re: Newbe question 35mm vs MF and dof
« Reply #110 on: January 05, 2011, 11:02:48 am »

Hi Nick,

Nice to hear!

Looking forward to your next message ;.)

Best regards
Erik

Hang on, David is actually right, not generally, but specifically. It suddenly dawned on me what he was on about, and I always had a niggle at the back of my mind that I was missing something.

Look, it's really late here, I'm off to bed but I'll tell you what Davids arguments really mean tomorrow...I don't mean to tease but I need to marshall my thoughts a bit more.
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Re: Newbe question 35mm vs MF and dof
« Reply #111 on: January 05, 2011, 12:51:26 pm »

Remembering Christopher original question:


I've had the same question in mind, specially "how will the dof compare?" between 35mm and MF.   I've been using a H4D-40 for several months, and I had the D3x (now only the D700) and I still have doubts regarding the DOF.

I have and example for Christopher, please correct me if I'm wrong.



Thanks for the example. However, it sort of misses the point. Don´t know if you can answer, but since you actually have made the whitch to MF:

 I am contemplating a switch from 35 mm Nikon to MF Leica. I intend to use it basically the same way I use my current camera. Hence I will change focal length to achieve approx the same framing without moving. Having done that, I would like to know how DOF wold change. Hence , if I take the picture with 50 mm at f 9 with my Nikon, what aperture would I need to use on the leica with a 70 mm? I usually print on A2 and sometimes on A1 (24 inches on either long or short side).
From the discussion this has started I gather there is no simple answer  ;).  

Chriastopher

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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #112 on: January 05, 2011, 01:32:07 pm »

Actually, yes there is a simple answer. The S2 has a sensor that is 30mm x 45mm in size. This is 1.25x longer in each linear dimension than full frame 35mm digital.

This means that to get a focal length that shows an equivalent field of view, you will need to use a lens that is 1.25 times longer than its 35mm equivalent. So the proper comparison lens to a 50mm prime would be a 62.5mm lens. So it's not exactly "apples-to-apples" to compare the 50 and the 70.

If you match focal lengths based on the 1.25x rule and shoot the exact same picture with the same settings and make similar sized prints (within the resolution limits of the smaller camera) then the Leica will have 1.25x less depth of field.  Aperture stops run on a factor of 1.4x, so the rough difference between the two formats will be just under one aperture stop of depth of field. However, this only holds true when you are well inside the hyperfocal distance. If you shoot a subject at or near the hyperfocal distance, the DOF of the Leica will start to be noticeably less than the 35mm shot since the hyperfocal distance for the Leica and the 35mm are not the same (ie. the 35mm will hit hyperfocal distance sooner than the Leica).

This article does a good job of summarizing the differences, just use a factor of 1.25x instead of the 1.6x used to compare FF and APS-C.

http://www.bobatkins.com/photography/technical/digitaldof.html
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #113 on: January 05, 2011, 03:18:09 pm »

OK, try this for size...

There is only one point of focus, everything else is more or less out of focus and the region that is acceptably sharp is called the DOF. No problems with that, all agreed?

Close to the point of focus there will be points that are only very slightly OOF, ie small discs not actual points.

These discs can be smaller than the ability of the sensor to resolve. The sensor cannot resolve the difference between the true point of focus and those points close by. 

Eventually, the further from the point of focus, either away or towards the camera/observer, these discs will be big enough for the sensor to resolve and they will start to appear less sharp than the region closer to the point of focus.

I'm pretty sure that this is what David refers to as the intrinsic DOF that is only related to the sensor and is not affected by print size. This would seems to be true.

The print size only affects the DOF when the print is smaller than the largest that it can be printed (which is a subjective thing, probably around the 200dpi point), at which time that 'intrinsic' DOF is the same as the actual DOF shown by the print. Any smaller sized prints will progressively lower the resolution and effectively increase the DOF as shown in the print. It does not affect the baseline DOF which is sensor/film grain dependant.

David's mistake (if you can call it that) was not in his knowledge but in the way he explained the point. The rest of us were not listening hard enough. There was always something missing from the exchange of ideas and points of view, some disconnect that I could not put my finger on. It came to me last night out of the blue - must have been my subconscious chewing things over!

We were all right after all.
« Last Edit: January 05, 2011, 04:25:15 pm by Nick Rains »
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Nick Rains
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jeremypayne

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Re: Newbe question 35mm vs MF and dof
« Reply #114 on: January 05, 2011, 03:31:04 pm »

OK, try this for size...

There is only one point of focus, everything else is more or less out of focus and the region that is acceptably sharp is called the DOF. No problems with that, all agreed?

Close to the point of focus there will be points that are only very slightly OOF, ie small discs not actual points.

These discs can be smaller than the ability of the sensor to resolve. The sensor cannot resolve the difference between the true point of focus and those points close by. 

Eventually, the further from the point of focus, either away or towards the camera/observer, these discs will be big enough for the sensor to resolve and they will start to appear less sharp than the region closer to the point of focus.

I'm pretty sure that this is what David refers to as the intrinsic DOF that is only related to the sensor and is not affected by print size. This would seems to be true.

The print size only affects the DOF when the print is smaller than the largest that it can be printed (which is a subjective thing, probably around the 200dpi point), at which time that 'intrinsic' DOF is the same as the actual DOF shown by the print. Any smaller sized prints will progressively lower the resolution and effectively increase the DOF as shown in the print. It does not affect the baseline DOF which is sensor/film grain dependant.

David's mistake was not in his knowledge but in the way he explained the point. The rest of us were not listening hard enough. There was always something missing from the exchange of ideas and points of view, some disconnect that I could not put my finger on. It came to me last night out of the blue - must have been my subconscious chewing things over!

We were all right after all.

In other words ... are you saying that David's DoF is simply the "maximum possible" DoF given a specific capture?

 
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Bart_van_der_Wolf

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Re: Newbe question 35mm vs MF and dof
« Reply #115 on: January 05, 2011, 03:53:18 pm »

OK, try this for size...

There is only one point of focus, everything else is more or less out of focus and the region that is acceptably sharp is called the DOF. No problems with that, all agreed?

Close to the point of focus there will be points that are only very slightly OOF, ie small discs not actual points.

These discs can be smaller than the ability of the sensor to resolve. The sensor cannot resolve the difference between the true point of focus and those points close by. 

Eventually, the further from the point of focus, either away or towards the camera/observer, these discs will be big enough for the sensor to resolve and they will start to appear less sharp than the region closer to the point of focus.

That's what I mentioned as resolution, but not DOF. The calculation of DOF requires a COC. COC depends on (angular) resolution which involves output magnification and a viewing distance. Different magnification/viewing distance changes DOF, and the circle is round. Even after capture, the COC remains a variable, so DOF cannot be a fixed quantity.

Quote
I'm pretty sure that this is what David refers to as the intrinsic DOF that is only related to the sensor and is not affected by print size. This would seems to be true.

I also think that's what he was thinking of, however DOF is a limiter of Resolution, but then so is diffraction. Resolution, or rather MTF, plays a role, but there is no such thing as an intrinsic DOF (which supposedly is to be unaffected by magnification). DOF requires a COC to be able and calculate it.

Sorry,
Bart

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Sheldon N

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Re: Newbe question 35mm vs MF and dof
« Reply #116 on: January 05, 2011, 04:17:43 pm »

Yes, because the sensor/film is resolution limited there reaches a point that you can no longer logically choose a smaller CoC, because the sensor/film cannot record it. This would be the smallest possible CoC, and minimum possible DOF (thinnest DOF possible), equivalent to the resolution limit of the sensor/film.  This all makes sense and we all agree with David on this technical point.

Where our opinions diverged was simply on whether choosing this "smallest possible, resolution limited CoC" was actually the true definition of DOF, rendering all other definitions of DOF incorrect. David argued that the minimum possible DOF was the absolute standard, and we argued that DOF was variable based on output and viewing conditions (ie DOF varies with print size).

All in all, just a debate over the semantics of what the definition of "DOF" was, and as David pointed out the definitions of DOF in the scientific community apparently are different than what is widely used in the photographic community.
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Nick Rains

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Re: Newbe question 35mm vs MF and dof
« Reply #117 on: January 05, 2011, 04:19:41 pm »

DOF requires a COC to be able and calculate it.
Sorry,
Bart

Certainly, and in this case the appropriate CoC is closely related to the resolution of the sensor, somewhere between the pixel pitch and some other larger figure depending on other variables like distraction and I suppose, lens quality. This choice of CoC can easily be proven by working out the max size of the print you can make from a capture using plausible specifications such as 200-240dpi then applying the normal DOF calculations based on this print's size and a 'normal' viewer at a very close viewing distance (allowing for close scrutiny). You will end up with a CoC which is roughly twice the pixel pitch. Close enough for me.

Using the Alpa spreadsheet and working with a print of 37.5" on the long side (7500px / 200) I require a CoC of 0.010 which is slightly less than twice the pixel pitch.

The magnification aspect is a bit misleading in some ways; there is essentially a lower limit on the DOF, and this only shows up at the maximum realistic magnification, the lower resolution of smaller prints simply obscures the DOF captured by the sensor which then looks like increased DOF. As I have always said this is an illusion (like so many aspects of photography), but no less useful for being an illusion.
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Re: Newbe question 35mm vs MF and dof
« Reply #118 on: January 05, 2011, 04:22:55 pm »

All in all, just a debate over the semantics of what the definition of "DOF" was, and as David pointed out the definitions of DOF in the scientific community apparently are different than what is widely used in the photographic community.

Not really. The definitions remain the same, the disconnect was the relationship between resolution and DOF. The scientific community agrees with the photography industry, or is it vice versa, the only difference is the use to which the phenomenon is put. Scientists are not interested in print making!
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Nick Rains
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Re: Newbe question 35mm vs MF and dof
« Reply #119 on: January 05, 2011, 04:26:12 pm »

In other words ... are you saying that David's DoF is simply the "maximum possible" DoF given a specific capture?

No, the minimum possible.
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