View Full Version : Why do lenses stop down so far?

Edward (Halifax,NS)
7-Jun-2004, 11:15
I just purchased a lens in a barrel mount that stops down to f/180. It is a 240mm APO Ronar. Why would anyone use settings that small? Would diffraction be horrible long before then? I have heard of people using f/64 when they are contact printing, but f/190 seems rediculous.

David R Munson
7-Jun-2004, 11:29
Sometimes the situation means that nothing else will work. I've shot some still life when the only stop that would work was AWD - all the way down. As far as it would go, past the last marked f-stop. Diffraction was there, but not as bad as I was expecting.

Thilo Schmid
8-Jun-2004, 01:29

on 8x10 with an admittable COF (Circle of Confusion) of 0.2mm (standard: format diagonal / 1500), the limiting aperture would be f300(!). The optimum aperture for 8x10 (i.e. where DOF and sharpness are the best compromize) is f120 for 8x10. Between f120 and f300, DOF is getting larger, but overall sharpness is reduced. Beyond f300, diffraction would be larger than COF.

The standard COF is of course an anachronism. But even with higher demands and a COF of 0.1mm, the limiting aperture would be f150. So f90 is not rediculous at all, even for 4x5 and not-so-high demands.

The real problem is, that you usually need more DOF for macro work and "macro" is reached fast on 8x10. The effective aperture is two stops less than dialed at 1:1.

Marc Genevrier
8-Jun-2004, 02:03
Edward, another way to put it is to calculate the resolution limit as imposed by diffraction for different f-stops.

Here are the approximate values I use:

f22 : 64 lpm (line pairs per mm)

f32 : 45

f45 : 32

f64 : 22

f90 : 16

f128 : 11


Note that for sake of clarity, the approximate values simply are the f-stops values in reverse order. You just have to remember one to know them all.
Assuming the eye can see about 5 lpm at a 25cm viewing distance (or 2,5 lpm at 50cm, etc.) it is easy to determine how big your print can be. For example at f128 for viewing at 50 cm : 11 divided by 2,5 equals about 4, so you can enlarge your original 4 times - which makes a big print if you're working in 8x10!


Marc Genevrier
8-Jun-2004, 02:04
I forgot one thing: The f-stops in my calculation are the effective f-stops, that means that they are corrected for bellows draw.

David A. Goldfarb
8-Jun-2004, 03:02
Those small f/stops are usually found on process lenses not intended for conventional pictorial photography. I would guess that for that kind of work, there are applications where diffraction is not a big issue.