Diffraction large format vs smaller

[George Kara]

There appears to be a difference when diffraction lowers the performance of a LF lens at higher apertures vs. smaller formats. I assume it is because of the larger negative therefore the effect is spread out over a larger space but really dont have a clue.

Does anyone know this answer?

[BradS]

diffraction happens at different f/stops for different focal lengths because f stop is a function of focal length. Diffraction effects are directly related to the size of the slit (or aperture) through which the light is passing...

[BetterSense]

I believe that the amount of diffraction in terms of circle of confusion is constant for any given f/stop, regardless of the camera format, but that the larger circle-of-confusion of largeformat, allows this to be less of a problem due to the reduced enlargement that the image undergoes for final viewing.

I could be wrong though.

[BradS]

I believe that the amount of diffraction in terms of circle of confusion is constant for any given f/stop

This is incorrect (see above)

the larger circle-of-confusion of largeformat, allows this to be less of a problem due to the reduced enlargement that the image undergoes for final viewing.

Yes, this is partially correct but it is not the full explanation of why diffraction effects become apparent at different f/stops for different focal lengths (and therefore it appears, formats).

[ic-racer]

There appears to be a difference when diffraction lowers the performance of a LF lens at higher apertures vs. smaller formats. I assume it is because of the larger negative therefore the effect is spread out over a larger space but really dont have a clue.

Does anyone know this answer?

If you consider the absolute aperture size in millimeters, then the relative enlargement of each format is an equalizer. So, diffraction is essentially the same across format sizes for the same print size and the same aperture size in millimeters. This is mathematically derived from the sizes of 'bullseys of confusion' produced by diffraction through the aperture.

[Jeff Keller]

One of the best write ups I've seen is:
http://www.cambridgeincolour.com/tut...hotography.htm
Play with the calculator near the bottom of the page.

For me the bottom line is:
aperture matters, focal length doesn't with fixed film size (ignoring DOF)
printed size matters
if DOF is inadequate or barely adequate, LF may be no better than 35mm, and diffraction can't be reduced by using a wider aperture

The advantage of LF (available when scene has limited DOF) comes from the fact that any given focal length covers a wider view and thus less enlargement is used to get the final print size.

Jeff Keller

I believe that the amount of diffraction in terms of circle of confusion is constant for any given f/stop, regardless of the camera format, but that the larger circle-of-confusion of largeformat, allows this to be less of a problem due to the reduced enlargement that the image undergoes for final viewing.

I could be wrong though.

[Jeff Keller]

I believe BetterSense is correct. The larger physical aperture of the longer focal length causes less angular divergence but the light travels further before forming the image, resulting in the same distance spread at the image plane.

See:
http://www.cambridgeincolour.com/tut...hotography.htm

Jeff Keller

Quote:
Originally Posted by BetterSense
I believe that the amount of diffraction in terms of circle of confusion is constant for any given f/stop

This is incorrect (see above)

[Glenn Thoreson]

I wonder what the f/64 group would say about this.

[Mike1234]

IIRC, at least some of the f/64 group changed their methods.

[Nathan Potter]

Diffraction (properly Fraunhofer diffraction) is the result of light rays having to travel a slightly longer path length from the edge of the aperture opening than from the center of the aperture opening thus interfering at the image plane causing a radially diminishing set of dark lines. The diameter of the first dark interference pattern is called an Airy disc diameter and can be computed mathematically as D = 2.44(lambda)N. D = diameter in microns; lambda = wavelength in microns, N = f/no. So an f/5.6 lens of any focal length produces the same size airy diameter. It is the Airy diameter that is the diffraction limit.

To place the above relation in a comprehensible fashion (good luck) one needs a picture of how a subject image is resolved at the film plane. This need have nothing to do with a lens but only requires a pinhole, as in a pinhole camera where the pinhole is equivalent to the aperture. Note from the equation above as the pinhole gets larger (larger f/no) the diameter of the airy disc gets larger. The key here is conceptual. Every part of the pinhole opening sees each individual resolvable point in the subject and delivers it to the film plane with an airy disc diameter determined by the size of the pinhole only. Thus the number of resolvable points in the subject is determined by the size of the opening. And, if you will, every one of these resolvable points has its own diffraction limit. If we rewrite above so that D = 2.44(lambda)Fl/A in place of f/no we see that holding the focal length (Fl) constant and increasing the A (diameter of the aperture) we decrease the diameter of the airy disc and conversely if we hold A constant and increase Fl we increase the diameter of the airy disc. In the case of large format lenses the diameter of an airy disc is no different than with small format lenses but of course one has a much larger image to work with.

So the OP is on the right track - it's the larger negative that is golden with LF. However, LF maximum apertures are usually around f/5.6 rather than some at f/1.2 with miniature cameras. And of course microscope objectives with mm focal lengths can achieve very small Airy disc diameters especially using oil or DI water immersion (of course with a lens, not a pinhole).

Nate Potter, Austin TX.