Diffraction and using a small aperture

[LF_NLD]

Hello everyone.

I have a technical question: in LF photography using really small apertures of f/64 is not uncommon. How does this affect the sharpness of the photo, thinking about diffraction?

And another question. The f-stop is related to the focal length. When you use a 210mm lens on an aperture of f/64, you have an opening equal to 210/64 = 3.3 mm. If you want the same opening on a 18mm lens (e.g. on a small format camera), you need to use 18/3.3 = f/5.4. And that's not at all a large number. So how can you possibly make sharp images using LF ... By using movements ofc!

Ok, I just answered myself. A 3.3 mm opening is not at all that 'closed' to cause diffraction, and movements do the rest. I still wanted to post this anyway.

[Bruce Barlow]

Call me blind, but I have never seen an image that was degraded in any meaningful way by diffraction. In other words, the gain in depth-of-field far outweighs the degradation from diffraction.

At least, that's my experience.

Rats! I've misplaced my white cane!

[vinny]

I recommend you search the forum and read the numerous threads on diffraction. It's a dead horse that's been well-beat.
Here's 10+ threads to read.

[djdister]

This information from Rodenstock has some interesting conclusions - basically they just say that the "Best Aperture" to use decreases as the film size gets larger, somewhat regardless of the focal length.

Here is the table from the below article:


Here is the full article:
DEPTH OF FIELD VERSUS DIFFRACTION - THE OPTIMUM WORKING APERTURES
http://www.prograf.ru/rodenstock/lar...en.html#table2

[jp]

Also not mentioned is that with LF, you either don't enlarge or enlarge by a pretty small multiplier compared to the small formats. A contact printed made on photo paper is going to lose some potential detail as the paper is not as detailed a medium as film.

[ic-racer]

How does this affect the sharpness of the photo, thinking about diffraction?

The effects of diffraction are quite visible. Just expose some large-format images at very small apertures and see for yourself. (eg smaller than f90 for 8x10). If you don't see the effects at the smallest settings on your lenses, be grateful that the burden of thousands of internet posts on the subject has been removed from your back.

[Ken Lee]

To see it for ourselves, we merely need to place an inexpensive loupe on the groundglass and focus on a subject having distinct edges, then gradually stop-down the lens.

Resolution and contrast reach a peak within a certain narrow range of apertures, after which they decline.

My experience matches the Rodenstock numbers exactly, which should come as no surprise: they're the optical physicists after all.

[Emmanuel BIGLER]

I like very much this executive summary about diffraction-related posts,
as stated by Leigh on this forum:

Diffraction limiting is one of those threats that mothers use to get their kids to eat Brussels sprouts, as in:
"If you don't eat your sprouts you'll be diffraction limited for the rest of your life."

P.S. and the numerical value to remember in addition to Leigh's executive summarry, is that a perfect lens stopped down to f/N cannot deliver details smaller than about N microns, and this is totally independant from the focal length.

[Jim Jones]

To quickly observe the degrading of an image due to diffraction, observe the image in an enlarger through a fairly high power grain focusing aid while varying the enlarging lens aperture. These lenses usually limit the minimum aperture so photographers can't screw up by using too small an aperture, but they still stop down enough that one can observe fine grain becoming mushy.

The effect of diffraction in photographs should be evaluated more as an artist than as a scientist. My Cole Weston print of his father's Shell, 1927, seems diffraction limited. Nowhere does it appear perfectly sharp. There is no reason why it should; that photo is about form and luminosity, not acutance. A very small aperture was required to balance depth of field with sharpness. Most other photographs demand a wider aperture as the graph Dan posted suggests.

[Drew Wiley]

Once you start making big precise enlargements it can become quite an important factor. Many LF photographers never understand this for several reasons. 1),
they never make big enlargements; 2) their film plane is never flat in the first place, because a typical filmholder doesn't even have that standard of accuracy
(at least in larger sheet sizes, certainly 5x7 up); 3) they might be contact printing or not have ideal darkroom enlarging techniques to begin with; 4) maybe they've never actually seen a really sharp print. If you've ever seen some of EW's or even AA's classic ole negs blown up, sharp they ain't - pretty much mushballs, in fact. EW intended his work to be mostly contact printed, AA's held up to 20x24 or so (approx to only a 3X enlargement from 8x10) ... after that, then you can understand why he taught that big print are "supposed to be" printed on matte paper less contrasty, otherwise his own looked like hell. ... Now in the real world, there are always tradeoff with depth of field for the sake of uneven focus planes, shutter speed question cause things don't always hold still, on
n' on. I really don't even give a damn about the math - I'm concerned with what shows up in real prints. If it's a subject that I might potentially print quite large,
then I get paranoid about this subject, if not, then I have more options. But I never like stopping 4x5 down smaller than f/32, or 8x10 lower than f/64, or even
f/45 for fussier projects.