Lens Design For Maximum DOF

[robc]

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.

note from my earlier post that it is not only the size of the exit pupil that is important but also its distance from the film. The closer it is to the film the less the depth of field. So for two lenses of same FL at same fstop with same size exit pupil, the one which has the exit pupil closer to the film will have less DOField.

Did your measurements include the distance to the film?

[Helen Bach]

...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...

Depth of field is an object-space property (entrance pupil applies), depth of focus is the conjugate image-space property (exit pupil applies).

Best,
Helen

[robc]

Depth of field is an object-space property (entrance pupil applies), depth of focus is the conjugate image-space property (exit pupil applies).

Best,
Helen

Yes, but it has to be rendered on the film and the size and distance of the exit pupil will affect anything which is not in the object plane of sharp focus or am I missing something here.

[Helen Bach]

Yes, sorry. My previous post was a little too curt and not well considered in the circumstances. I was just trying to reiterate/restate what has already been said about the relative importance, or unimportance, of the pupil factor with the lenses under discussion here, especially for distant work.

Rather than using a design with a pupil factor of less than unity - which only gives significant increase in depth of field when focused close - the classic method of producing apparent increases in DoF by optical means has been to undercorrect abberations.

Arthur Cox mentions another technique: that of changing focus (and focal length, to maintain constant magnification) either during one single exposure or during multiple exposures on the same piece of film. He mentions a B&L patent for a cine lens with an element that oscillates at high frequency, but I guess that was never produced. Cox goes into the effects of aberrations on DoF in a slightly more quantitive manner (but only slightly).

Best,
Helen

[Leonard Evens]

The distinction between depth of field and depth of focus is always confusing. It is not only a matter of whether you are discussing object or image space but also which question yo are trying to answer. If you asume that you are perfectly focused on a specific subject plane in object space, you may ask for the region in object space which is still in adequate focus subject to a criterion for how out of focus it may be before you notice it (under specified conditions). That is the depth of field question. On the other hand, you may assume that you are interested in just one plane in object space with its corresponding image plane in image space, and you may ask how far the film plane can vary from the exact image plane and still have a sharp image of the subject plane, again relative to some criterion for sharpness. That is the depth of focus question. But in either case, you may consider the conjugate planes in either image or object space. For depth of focus, it is not usually interesting to translate everything to object space, but for depth of field it often is useful to look at what is happening in image space. In either case, the mathematical analysis is done initially in image space.