Shutter f/stop question

[timbo10ca]

The smallest f/stop marked on my (modern) copal shutter is 64. I can move the lever past it an equivalent distance as that between the 45 and 64 marks, and I can see the aperture close as I do this. Does this mean that my maximum usable f/stop is actually 90?

Tim

[Jim Noel]

Possibly.
The actual aperture may be smaller or larger than f 90. You will have to measure it and this is most easily done with a spot meter on the ground glass.
Even if it is f 90, it may not be usable because it may be so small as to cause diffraction across the edges of the blades producing a fuzzy image.

[Archphoto]

For most 4x5 inch optics f:22 is the best aperture

[ignatiusjk]

For most 4x5 inch optics f:22 is the best aperture

Why f.22? I've never heard of a single f. stop being optimal.

[John T]

Because both 11 and 22 are so gosh darn symmetric and 22 is twice as good as 11 :p

[Gene McCluney]

A lens has a point beyond which the resolution starts to degrade due to diffraction. Maximum resolution in most Large Format lenses is achieved at f22, and as you stop down beyond that you lose resolution, but you gain depth-of-field, which may be more important to you, since large-format negatives are not enlarged as much as small format negatives are.

Each aperture setting is a trade-off in some parameters. Wide open, lenses tend to have more flaws, which some people like, particularly for portraits, and very shallow depth of field, as you stop down the image keeps improving until about f22, then there is a gradual loss of resolution (you may not see it in most pictorial subjects), as you go beyond f22.

[Bjorn Nilsson]

Gene describes this very well. Now, what is diffraction?
When the light passes the edges of the aperture blades, it isn't just cut off or let past. The light which passes just at the edge will bend off a bit, causing a spread of the image instead of it being "perfectly" cast on the film.
Diffraction is something mechanic, which cannot be overcome by any different lens construction etc.
There is a enough information available on the 'net, so make a search for it.
What it all boils down to is that at f/16-22 the balance between optical performance and diffraction is at its best. If you got a sharp lens this is where it's really sharp. (Read e.g. S. Super Symmar, R. Sironar-S, process lenses etc.) At f/45 the optical performance is probably even better, but diffraction kicks in so hard that just about any lens as long as it contains glass will perform equally good (read "bad"). At f/90 I think the resolution is 1/4 of that at f/45.
You can compare it to a very good wine. Having one glass is f/22. f/45 rather means 2 bottles. f/90, a barrel. Why the wine comparison? The one glass tastes very good all way through. If you drink 2 bottles, you can just as well have the $5 wine, as it tastes the same after 2 bottles anyhow. At f/90, well...

//Björn

[Kuzano]

You can compare it to a very good wine. Having one glass is f/22. f/45 rather means 2 bottles. f/90, a barrel. Why the wine comparison? The one glass tastes very good all way through. If you drink 2 bottles, you can just as well have the $5 wine, as it tastes the same after 2 bottles anyhow. At f/90, well...//Björn

How many barrels of wine to test three lens/shutter combinations, and over what period of time should I spread the tests? Also, do I need a partner for the wine part?

[Bruce Barlow]

Try a test negative using an exposure setting for f90 based on what your meter tells you. Proof it with a known properly-exposed-and-developed negative. Do they match? Voila. If not, try again adjusting for what the test neg looks like (it's either too light or too dark or juuuust right). The second negative will be close enough. Declare victory and go photograph.

If you find out for yourself, you'll know it for life, and learn other things in the process.

And fuhgit about diffraction unless you make wall-size enlargements. You won't see it in normal life. It barely was visible, when I was looking at someone's hugely-enlarged tests, in specular highlights and photographs of light bulbs. Make a lot of those, do ya?

I think depth-of-field considerations greatly trump worries about diffraction. Gimmee f90!

[Chauncey Walden]

Anyone ever hear of the "Rule of 4"? It stuck in my mind but I have no idea of the source. It went something like "divide the focal length by 4 and that is the aperture beyond which diffraction becomes objectionable in the image." Of course, what it is saying is that an effective aperture smaller than 4mm isn't as good image wise as a larger one. It sort of fits in with "general" wisdom: 4x5 (150mm) f/32; 5x7 (210mm) f/45; 8x10 (300mm) f/64; 11x14 (360mm) f/90. Of course, for a 90mm lens that would mean f/22 max. Any optical gurus out there with input?

[racenviper]

I have known for years of shooting 35mm film that the lens was it sharpest at F8 to F11. I always went for the fastest lens and the aperture did change in the zoom range.

New to LF I have noticed the same, f22 has good sharpness on the neg.

But that just me.