View Full Version : Generic Lens Performance Question

Robert McClure
4-Mar-2005, 10:02
A current ebay ad lists a 300mm Nikkor-W as yielding 420mm image circle at f22. I really don't understand a whole lot of issues related to this. Can any of you folks out there help me a bit here?
Many thanks in advance!!!

1. Imagine I project an image generated by the lens onto a huge sheet of groundglass. I'm assuming f22, infinity, acceptably sharp focus, and critical viewing of image with a loop.

2. So I get my 420mm of useable/good quality image circle which the manufacturer is promising in his, I assume, conservative specs.

3. My generic question about lens performance: as I stop down from 22 toward 64 or from 22 toward wide open, what happens optically? What happens to my useable (above) area? Will it tend to lessen in size (bigger/ smaller?), degrade in quality, or what?

4. Put another way, folks/manufacturers will say of a particular lens, "covers xxx at so-and-so f-stop at infinity." Does that mean that at other than the quoted/published f-stop (like the common f22 above), the total projected image circle is literally smaller or larger? Or does the image circle physically stay the same size but with major quality changes within the, say, "420mm image circle."

Just trying to learn. What primer books might I check out? Many thanks!!!

Robert McClure - Atlanta

Ralph Barker
4-Mar-2005, 10:30
The image circle is the portion of the projected image (circle of illumination) that is acceptably sharp. Image sharpness almost always improves as the lens is stopped down from wide open, the focus/composition aperture, and with some lenses, the image circle gets considerably larger, as well. F22 or f16 are commonly used for the image circle specs to give a point of comparison at a reasonable shooting aperture.

There are several articles about lenses on the main page of this site that may help you with lens design and performance questions. (see the link at the top of the page)

Robert McClure
4-Mar-2005, 11:18
Thanks, Ralph!

Robert McClure

Melchi M. Michel
4-Mar-2005, 12:01

In addition to increasing the size of the acceptably sharp region of the circle of illumination (as Ralph noted above), stopping down the aperture generally increases the size of the illumination circle itself. This is because, in addition to optical vignetting (AKA pinhole vignetting, which falls off approximately as cos(x)^4 ) , any lens whose depth is significant relative to its physical aperture size also suffers from mechanical vignetting. This vignetting becomes less significant at smaller apertures because the angle from the edge of the aperture to the edge of the physical lens becomes greater, allowing a wider angular range of incident rays to pass through the aperture.

Kerry L. Thalmann
4-Mar-2005, 12:02
Image sharpness almost always improves as the lens is stopped down from wide open, the focus/composition aperture, and with some lenses, the image circle gets considerably larger, as well. F22 or f16 are commonly used for the image circle specs to give a point of comparison at a reasonable shooting aperture.

This is true, sort of, to a certain extent. More accurately, corner performance general gets better as you stop down. However, center sharpness actually starts to degrade for most LF lenses once you stop down beyond f11 or f16 due to diffraction. The fact that most LF lenses are specified at f16 or f22 is that these apertures usually yield the best overall compromise between center and corner sharpness.

Stop down too far and at some point all lenses become pretty much equal in terms of sharpness - they become diffraction limited. Stopping down "too much" varies depending on other variables. If you're contact printing an 11x14 negative, diffraction is less of an issue than trying to make an 11x14 print from a 35mm negative. f64 may be a perfectly reasonable shooting aperture for 8x10 or 11x14, but shoot 35mm at f64 and try to enlarge it to 11x14 and you'd get a very unsharp result. Medium format and 4x5 fall in the middle of these two extremes. I almost always shoot in the f16 - f32 range (F22 +/- one stop) on 4x5. However, sometimes, even with camera movements, I need to stop down to f45 to get the desired depth of field for a particular shot. Such is the nature of recording a three dimensional world onto a two dimensional surface.

With regards to coverage increasing as you stop down, this is a function of both optical and mechanical design. Some lens designs will have greater usable coverage as as you stop down, others will not. Also, most modern lenses incorporate field stops to limit the image circle to what the manufacturer considers acceptable sharpness. Older lenses did not incorporate field stops. The result is that they often throw huge image circles, but only the center portion is acceptably sharp. The circle of sharp definition will continue to grow as you stop down and may eventually approach the circle of illumination. The classic Goerz Dagor (and derivatives like the Schneider Angulon) is an exampkle of such a lens. A Dagor will have a useable coverage of about 65 degrees at f22 (the definition of "useable" is subjective), but will increase to about 87 degrees at f22. With a few exceptions (G Clarons and the Nikkor M series, for example) most lenses made in the last 30 years won't provide a significant increase in useable coverage beyond the specified image circle at f22 - maybe a few millimeters before the field stops mechanically limit the circle of illumination.


Emmanuel BIGLER
4-Mar-2005, 14:25
Robert. In terms of illumination of the image field, you can easily check yourself a basic phonomenon named "cats'eye". Open you view camera lens to its widest aperture and look at the image of the iris from the corners of the ground glass, or without any camera simply look at the exit image of the iris (=exit pupil) seen far from the optical axis. You'll see that the exit pupil has the oval shape of a cat's eye beyond a certain angular distance from the optical axis, due obstruction by lens mounts. Stop the lens down and from the same place where a cat's eye could be seen, now you see a small circular aperture, not obstructed by any lens element mount.

So a first reason why the illuminated circle increases when stopping down is related to the cat's eye effect. Besides this, image quality increases at the centre and in the egdes to certain extent when stopping down. Older lenses became fuzzy in the corners before light fall-foo due to the cat's eye effect could be noticeable.

Modern view camera lenses designed for LF film cameras have a circle of good sharpness almost as large as a circle of even ilumination, the contour of the useable image field being very small, a few millimetres, with an abrupt drop both in availale light and image sharpness.