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View Full Version : Why do Apo Nikkors stop down so far?

Robert McClure
26-Jan-2005, 08:52
I would really like for someone out there in the ULF world to respond to my question: Why do some process lenses like the APO Nikkors commonly used by ULFers, stop down to, say,
f 128? I know generally about the phenomenom of diffraction - and apertures as small as 128 would seem to introduce objectionable diffraction. Why then do some of the graphic arts lenses seen commonly today in the used market stop down so far?

I can hazard only one guess:

Is diffraction not an issue at, say, f 128, if what you are photographing is line copy onto huge sheets of litho film.

Many thanks,
Robert McClure
Atlanta

Nick_3536
26-Jan-2005, 09:28
Actually why do they stop down at all? My understanding is that all process lenses are designed to be used at one F/stop [depending on focal length]. I don't think they focus with the lens. So why not just make the lens F/22 or whatever it's going to be used at?

Emmanuel BIGLER
26-Jan-2005, 10:15
The best aperture, i.e. the aperture for which the image is at best taking into account residual aberrations and diffraction changes when you change from medium format to large format and to ultra-large format.
A simple model, assuming that aberrations scale like the focal length for a given lens formula, compared to diffraction which depends only on the relative f-stop (numerical aperture), suggests that the best numerical aperture "N_best" increases proportionnaly to the focal length or format diagonal. This idea is supported by a compilation of the best f-stop for standard lenses as published by manufacturers from 6x6cm to 8"x10" ; it can be shown that the best numerical aperture follows the approximate scaling rule : f_best = focal length (in mm) / (8mm).
For example in 6x9 with a 100 mm lens, the best f-stop according to this rule of thumb is N_best=12.5, in between 11 and 16. With a 150 mm in 4"x5", the rule yields 18.75 in between 16 and 22 ; in 8"x10" with a 300 mm you get 37.5 in between 32 and 45, etc...
So in ULF, if we extrapolate the rule, say : in 11"x14" with a diagonal of 450mm, expected N_best should be N_best= 56 in between 45 and 64.
From the best aperture you can stop down one or two stops more, the degradation of image quality due to diffraction is "smooth" but clearly visible at N_best+2 clicks..
So to me it is perfectly adequate to stop down a 450mm standard (50-70 degrees) lens for 11"x14" to f/90. F/128, 2-1/2 stops beyond the best expected f-stop of 56 is probably not to be recommended.

Emmanuel BIGLER
26-Jan-2005, 10:25
I agree with Nick for dedicated process lenses, some of them were actually equiped with a fixed iris designed for the best numerical aperture N_best. N_best=22 is valid, say, for 150 to 250 mm process lenses for 4"x5" ; longer focal lengths for larger formats will need to increase N_best proportionnaly to the focal length for a given lens design.
People who used those lenses to copy flat artwork only wanted the best image quality and did not need depth of field adjustments ot photometric adjustments of the iris. But photographers who want to re-use those excellent lenses would be embarassed with a fixed iris outdoors, recording 3-D scenes.
Classical process lenses are probably no longer in use nowadays, but modern top-class lenses designed for wafer steppers have in principle a fixed numerical aperture in ordre to deliver "just" diffraction-limited images.

Eric Woodbury
26-Jan-2005, 10:54
I have the 300/9 lens and it stops down to f/128. I have actually used it there. As you noted, because of the diffraction limit, it is not a good performer there. Lenses perform their best at a couple of stops down. We'll just leave the theory there. As the lens is stopped way down, it is acting more and more like a pinhole lens. Pinholes have their advantages, too -- besides cheap, they have the same point of confusion size at any distance. This is why you might want to stop down, way down, is to overcome depth of focus problems. If you want near and far in focus at the same time, there is no other way to do it other than to stop down. You are balancing depth of focus and diffraction (and if the wind is blowing -- motion).

There is a wonderful article here at LFphoto info that describes how to get the best performance from a lens for a given depth of focus.

http://www.largeformatphotography.info/fstop.html#ref (http://www.largeformatphotography.info/fstop.html#ref)