Lens Design For Maximum DOF

[Scott Rosenberg]

i have a bit of an academic question for the optics experts in the group. i was recently discussing using process lenses for infinity work with someone who is quite knowledgeable on such matters. we were going back and forth on the merits and limitations of the lenses, noting the usuals...

Pro: Lenses are small and usually very sharp at f/22 and beyond
Cons: Lenses are not designed for infinity use, and need to be stopped down a lot to get acceptable results at infinity

he then said something that i'd not heard or even thought about before... that process lenses, we were specifically talking about the APO-Ronar, which is, i believe, a dialyte, have shallower DOF at a given f-stop than lenses of other designs. i told him that Linos could adjust the spacing to optimize the APO-Ronar for distant shooting, but he said that would only enable one to shoot the lens with acceptable results at f-stops below f/22, it would not correct the narrow DOF inherit to the lens design.

he held that due to the lens design, an APO-Ronar that was modified for distant work, would still have less DOF at the same f-stop as a different design, say a plasmat or tessar.

is there any truth to this? does that mean that the fuji 300-c, which is essentially a dialyte optimized for infinity work, will have less DOF than a 300mm lens of another design at the same f-stop?

whether this amounts to any real difference in use is a separate argument altogether; i'm just wondering about the theory.

[Armin Seeholzer]

I'm not a lens expert but it still does not make sence to me.
The Ronars which came from the factory with shutter are optimised at 1:20 and not at 1:1 and for the others Linos is giving a service if someone likes it to optimise them at infinity!
This was told from a Linos expert!

[Oren Grad]

In classical geometric optics, DOF is a mathematical consequence of the parameters you feed the equation - for a given focal length and focus distance, plus whatever assumptions you want to make about visual acuity of the observer and consequently about the applicable circle of confusion, the depth of field is unambiguously determined.

But in his "Applied Photographic Optics" text, Sidney Ray has an interesting discussion about departures from the pure geometric theory. He mentions the following situations where the effective DOF may differ materially from the calculated values:

* Cases where the exit pupil shape is markedly non-circular (he mentions AE cameras with 2-bladed, odd-shaped diaphragms; the Minox 35 is one specific such case that I'm aware of)

* The presence of uncorrected coma, which gives a marked asymmetry to the blur spot

* Undercorrected spherical aberration, which the "circle of least confusion" does not correspond to the classically-assumed shape or luminance distribution of a circle of confusion. As Ray observes, "with undercorrected SA, the foreground is better defined than the background, which is the reverse of the usual practical requirement".

As an extreme case, he notes the soft focus lens, where aperture-dependent, undercorrected SA is used to give a soft effect with a "core" of sharpness, but as a consequence the blur circle cannot be defined and DOF calculations are pointless.

I'd add one point to Ray's discussion: from a purely subjective point of view, the character of a lens's bokeh - its out-of-focus rendering - can have an effect on the perception of DOF. For example, a lens that frizzes out (bright-ring, double-line, or "ni-sen" bokeh) very quickly as you move away from the plane of focus may have less apparent DOF than a lens that retains coherence of shapes farther out from the plane of focus.

I have no idea if any of this is what your friend had in mind in the specific comparative example he cited - he may not know himself. To make sense of that specific case, someone would need to be able to tell a story about systematic differences in the ways the two lens types trade off correction of different aberrations like coma and SA.

Nor do I know whether anyone has published "corrected" DOF tables that attempt to take into account these situations - the calculation is probably pretty nasty in such cases - or how frequently the deviation from theoretically ideal behavior is sufficient to make a practical difference. At least in principle, one can tell a story about how perceived DOF could be affected by lens design. As to whether the difference between any two particular lenses would matter to you, I'm afraid there's no alternative but to make your own tests, both because of the absence of standardized calculations to account for different aberrations and iris shapes and because of the subjective character of the perception of DOF.

[Dan Fromm]

Scott, I don't know where your friend got the idea that Apo Ronars aren't great at distance. The optics texts say that (a) a lens can be optimized for only one set of conjugates and (b) that dialytes -- that's what the AR is -- hold their corrections very well at all distances. If you go looking, you'll find propaganda from Rodenstock that make the point, and very firmly, that an Apo Ronar will give better image quality with a distant subject than any telephoto lens.

In general, and let's ignore the exceptions that Oren brought up as true but not relevant to what we do or the lenses we use, depth of field depends on relative aperture (f/ number) and magnification and arbitrarily selected circle of confusion. That's it. At the same magnification and f/stop and coc all lenses give the same depth of field. There is no magic way to get more DoF, even though all of us who do a lot of closeup and photomacrography wish there were one.

And this bit about process lenses being optimized for f/22 is hogwash, pure hogwash. Many of them reach their rated coverage at f/22, but in fact are sharper centrally wider open. Remember that the rated coverage is based on MTF at a "reasonable" resolution. I've just been through this exercise with a G-Claron and a couple of Apo Ronars. With all of 'em, f/22 is worse than f/16. Same with my Apotals, Apo Nikkors, Apo Saphirs, RF-5, GRIIs, ...

Stopping down reduces aberrations that are controlled by aperture, and these are worst at the field's margins. There's a character who posts a lot on photo.net who insists that curvature of field is the problem and that stopping down flattens the field. Not so, say my texts, stopping down kills coma. And that's why coverage (remember, adequate contrast at desired resolution) can be improved somewhat by stopping down.

If you have a long dialyte or any other long process lens, go shoot it and be happy.

Cheers,

Dan

[Jim Galli]

One obscure exception just to make you nuts. Long, long ago Wollensak marketed the "Beach Multifocal". It was an ordinary tessar except in the rear cemented group, one of the cemented surfaces was intentionally fresneled. Interesting lens. It was supposed to be a cake and eat it too for portraitists who needed to shoot wide open for speed but needed as much depth as possible while filling their 8X10 plates.

[Scott Rosenberg]

dan, i pretty much suspected what you are saying all along - BUT, my friend has a lot more expecience with such matters. i guess another way to ask this would be if you take several different lenses of the same focal length, each representing one of the various lens designs out there (dialyte, tessar, plasmat, etc) and focus them at exactly the same distance, then stop them down to exactly the same f-stop, would the area of sharp focus extend the same distance in front of and behind the point of focus for each of the lenses? sounds like from what you're saying, they'd all be the same.

[robc]

dan, i pretty much suspected what you are saying all along - BUT, my friend has a lot more expecience with such matters. i guess another way to ask this would be if you take several different lenses of the same focal length, each representing one of the various lens designs out there (dialyte, tessar, plasmat, etc) and focus them at exactly the same distance, then stop them down to exactly the same f-stop, would the area of sharp focus extend the same distance in front of and behind the point of focus for each of the lenses? sounds like from what you're saying, they'd all be the same.

not quite because each wavelength focuses at a different focal length unless the lense is designed to focus all wavelengths at the same FL. APO lenses are better than non APO lenses and hence will have a slightly greater DOF as I understand it.

If that APO lens has elements which get the wavelengths to focus at the same FL at 1:1 magnification, then adjusting the spacing of the elements to give optimum focussing at 20:1 magnification is likely to remove the APO designation from the lens, thereby making its DOF less than a lens designed to be APO at 20:1 magnification.

Having said that, process lenses are expensive because they use the best of everything with the most stringent manufacturing tolerances, therefore there may be some margin to adjust to being a 20:1 or whatever magnification and still be comparable with the best normal taking lenses.

As always, without knowing all the variables and exactly what you are comparing, it is mere speculation.

[Scott Rosenberg]

As always, without knowing all the variables and exactly what you are comparing, it is mere speculation.

rob, the precise scenario we were debating was whether a 300mm apo-ronar that had been optimized by linos for distant work or a 300mm fuji-c (dialyte like the ronar, but already set for distant work) would provide the same DOF as a lens of another design, say the tessar design nikon 300-m. take both of them, focus them at exactly the same point, stop them down to exactly the same f-stop - would the DOF be identical, or would the dialyte, by the nature of it's design, have a shallower DOF.

the argument was basically does lens design have any effect on DOF. i always thought variables independent of the particular design like focal length and aperture, were what determined DOF. this is why is was surprised to hear him make such a claim.

[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...

[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.