Originally posted by cats_five I hadn't considered that the absolute aperture size, not the f-stop, is what determines when defraction starts to be a problem. ......

Not exactly...

Diffraction spot size is determined by the ratio of absolute distance from the aperture to absolute aperture size. The Wikipedia article

Airy disk - Wikipedia, the free encyclopedia ( and any basis physics source) says...

"

**...where x is the separation of the images of the two objects on the film and **

f is the distance from the lens to the film..... x = 1.22(wavelength)f/d........"

When the lens is focused far away the focal length f is a good measure of the distance from the lens to the film, but not for close focus. The correct distance for all cases is (focal length)(1+magnification); therefore the appropriate relationship describing diffraction spot size is:

**Airy Disk radius = 1.22(wavelength)(f-number)(1+magnification)**
Therefore, as magnification increases, the f-number where diffraction becomes apparent decreases inversely proportional to magnification.

For a perfect lens, diffraction for macro photography will appear when:

Diffraction limit f-number for macro = (Diffraction limit f-number for infinite focus)/(1+magnification)

As our lenses are not perfect, the situation is usually worse than indicated by the perfect lens relationship above. Logic implies that for macros the f-number indicated on the lens should not be taken below the infinite focus "sweet spot" to avoid diffraction softening for any magnification. That effectively limits the maximum useful macro depth of field.