Lens Design For Maximum DOF

[robc]

alright, lets put it another way. The DOField is a function of not only the focal length, but also the size of the exit pupil and the distance of the exit pupil from the film. For lenses of the same focal length, the lens with the biggest exit pupil at the same fstop will probably have a narrower DOField.

So we are comparing the sizes of the exit pupils. Do you have the figures for the size of the exit pupils and all the other variables such as residual abberations etc etc.
No I thought not. So again, what are we comparing or are we merely speculating...

[Oren Grad]

Even if in theory tessars as a class have different DOF characteristics from dialytes as a class - a notion for which no specific evidence either empirical or theroretical has been presented here - good tessars and dialytes may be sufficiently well corrected overall that a theoretical difference, even if it exists, may not be practically meaningful.

If I had to guess, I'd speculate that either there is no class effect, or that there is a small one on average but that there's so much variation among different instances within types and so much overlap between the two distributions that it will be pointless to generalize.

At any rate, I think the only way you'll ever be able to answer your question conclusively is to do such a test and see for yourself.

[Scott Rosenberg]

So we are comparing the sizes of the exit pupils. Do you have the figures for the size of the exit pupils and all the other variables such as residual abberations etc etc. No I thought not. So again, what are we comparing or are we merely speculating...

rob, i will refer you back to something i mentioned in my original question... "whether this amounts to any real difference in use is a separate argument altogether; i'm just wondering about the theory."

this all amounts to pure speculation! he was hypothesizing something that contradicted everything i knew about lenses... so, i thought i'd post here and ask about his theory in general. any specifics i cited were either to present a more complete explanation of the debate.

[robc]

I don't know anything about the differences of the two designs but what I say holds true. The exit pupil size and its distance from the film is what could make the most difference if it is more significant than the other factors which have already been mentioned.

[Scott Rosenberg]

I don't know anything about the differences of the two designs but what I say holds true. The exit pupil size and its distance from the film is what could make the most difference if it is more significant than the other factors which have already been mentioned.

that's perfect... that's what i was looking for!! so, there is, in theory, some truth to what he was saying. even if only to a negligible degree, there are variables in the lens design that can contribute to the DOF. whether this is meaningful in pratical applications, i highly doubt, and enjoyed the thread a great deal.

thanks all!

[Michael S. Briggs]

The f-stop is defined as the focal length divided by the diameter of the entrance pupil. The pupil magnification is the ratio of the diameter of the exit pupil over the entrance pupil. So for two lenses at the same f-stop to have different exit pupil diameters, and thus make robc's idea relevant, they would have to have different pupil magnifications. Most LF lenses have pupil magnifcations close enough to one that the effect won't come into play. Probably the main exception is telephoto lenses.

If you look at the dof derivation in Sidney Ray's book, Applied Photographic Optics, he makes the approximation that the entrance and exit pupils are located at the principal planes. So maybe a lens that substantially violated one of these assumptions might have a somewhat altered dof as a function of f-stop. Again, among LF lenses, probably candidates are true telephotos.

On the subject of using spherical aberration to modify image characterics outside the plane of best focus, modern examples are the 35 mm telephotos from Nikon with the Defocus control. This adjustment is said to modify the spherical aberation to change the blur characteristics in front and behind of the subjects -- see http://www.nikon-image.com/eng/LensGuide/opt_tech2.html. This isn't exactly the same as modifying the depth of field.

[GPS]

Nowhere in Ray's book is it said that a dialyte typ of lens has a different dof than a plasmat etc. Also, as the dof in macrophotography is small by definition any differences (if they existed) between the different types of lens would be for all practical reasons, insignificant. It is not correct to speak about dof differences inherent to the lens design such as dialyte, plasmat etc.

[Emmanuel BIGLER]

If we go back to the original question by Scott Rosenberg related to DOF on apo-ronars and other similar lenses, my understanding is that used at its proper viewing distance i.e. the distance for which it has been optimised by any means, the apo-ronar is better corrected from aberrations @f/22 than most other vintage lenses.

So my understanding of the original experience as quotedby Scott could be that apo-ronars seem to have apparently shallower depth of field because at a time they were the only ones not affected by coma and residual spherical aberrations visible on other old designs. The father of the dialytes is the Celor and is as old as the tessar (beginning of the XX-st century). At the time many not-so-good lenses were in regular use by photographers.

In its optimum, perfectly symmetrical use at 1:1, with the iris located at the centre of symmetry, coma and lateral color are corrected by symmetry. Spherical aberration is corrected by the optical design itself. My understanding is that those inherent qualities degrade only marginally when departing from the 1:1 ratio, however a proper spacer ring or slight modifications of the design can change the optimum distance of course as quoted by Arne Croell in his article on apo germinars. Jena Guys knew their stuff ;-)

So as a summary, what about something like : only the apo ronar could satisfy the traditional geometrical DOF theory ;-) other vintage lenses did not.
Moreover dialytes being perfectly symmetrical, there is no question about where the pupils are, the classical DOF formulae apply.

As I see it, all modern lens designs being extremely well corrected will comply with the geometrical DOF theory, extended to non-unit pupillar magnification.
When the pupllar magnification ratio is non-unit the traditional value for the hyperfocal distance still applies and defines DOF limits for far-distant objects ; only at shorter distances will the formulae slightly differ from the symmetrical case. See the article by Mr. Van Walree.

http://www.vanwalree.com/optics/dofderivation.html

So except when using a telephoto or a retrofocus for close-up, again one may sleep well and continue to use the good ol' formulae without fear.

[Leonard Evens]

I think Emmanuel is substantially correct. I would add that the DOF formulas are derived assuming an ideal lens which can be described precisely in terms of cardinal points and reference planes. No real lens matches such an ideal lens, so the formulas can't apply literally to real lenses which must always differ in some way from predicted performance. But modern lenses may come very close. Even an ideal lens suffers from diffraction, so DOF formulas have to be used at small relative apertures with that in mind. With respect to entrance and exit pupils, the effect of pupil magnification is already taken into account in the exact formulas, as described, for example, in Jacobson's Lens Tutorial. Finally, I can't let the statement that DOF is independent of focal length stand. For relatively near subjects, with format fixed, if magnification is kept constant, DOF is close to being independent of focal length, but those restrictions don't always apply.

[Michael S. Briggs]

Several questions are being asked here.

Leonard pointed out a full version of the DOF equation that includes the pupil magnification, rather than ignoring this factor. One version on the web is at http://www.photo.net/learn/optics/lensTutorial. The pupil magnification p appears in the factor (1+M/p). There is a table of lens pupil values in Ray's book -- the values range from 0.44 for a long 35 mm telephoto to 3.5 for a fisheye. So if you focus at a large distance so that the magnification M is near zero, the factor (1+M/p) will become essentially one and the pupil magnification won't matter.

So for the question "for two lenses of the same focal length designed to deliver sharp images, can they be designed to give different depths of field at the same f-stop", the answer is that altering the entrance and exit pupils won't work for typical photography, only for closeup and macro photography.

Also, few LF lenses have pupil magnifications much different from one. I just made a eyeball / ruler measurement of an Apo-Ronar, tessar-type (Nikkor-M) and plasmat (Apo-Sironar), and they seems to have the same size entrance and exit pupils. (Probably some of these lenses have non-unit pupil magnifications at a higher level of measurement accuracy). So for the original question of whether an Apo-Ronar could have less DOF than a tessar or plasmat, the pupil idea probably won't work even for closeup photography.

The most likely LF lenses for pupil magnifications much different from one are telephotos.

The place in LF photography where pupil magnification (if different from unity) might be useful is in calculating exposure correction for closeups.

The other ideas, cited by Oren and Helen and myself, involve larger aberrations than would be normal in a modern, sharp LF lens. This would generally be considered too high a "price" to pay in a general purpose lens.