Diffraction: f-value or aperture diameter?

[well_dunno]

Hello everyone,

A question on diffraction: frequently  diffraction effect seem to be mentioned with reference to f-numbers, however, if I am not mistaken, diffraction effect negatively correlate with the diameter of the lens aperture and not with the f-number itself. As a result, everything else remaining equal, the effect should  be the same at 150 mm f/11 and 300mm f/22.

Just trying to get it clear for myself so any info/reflections are appreciated!

Cheers!   

[neuroanatomist]

Nope.  The diffraction effects correlate with f/number, not with iris diaphagm diameter.  That's because the distance between the iris diaphragm and focal plane also matters for diffraction.  To expand on your example, 300mm f/11 has an iris diameter twice as large as 150mm f/11, which you are thinking would decrease the effect of diffraction, but at 300mm that iris diaphragm is also twice as far from the focal plane, which increases the effect of diffraction.  So, iris diaphragm diameter and focal length effectively cancel each other out, and the f-number ratio is the determining factor.

[well_dunno]

Nope.  The diffraction effects correlate with f/number, not with iris diaphagm diameter.  That's because the distance between the iris diaphragm and focal plane also matters for diffraction.  To expand on your example, 300mm f/11 has an iris diameter twice as large as 150mm f/11, which you are thinking would decrease the effect of diffraction, but at 300mm that iris diaphragm is also twice as far from the focal plane, which increases the effect of diffraction.  So, iris diaphragm diameter and focal length effectively cancel each other out, and the f-number ratio is the determining factor.

 

Thank you! 

haha! This must be one of those situations WildBill was referring to in question and answer 9