Diffraction f/Entekaphobia

Started Feb 5, 2014 | Discussions thread
Bernard Delley Contributing Member • Posts: 829
Diffraction wideangle versu tele, laws of optics

Leonard Shepherd wrote:

If further testing is done it is likely to confirm the diffraction effect is usually strongest with a wide angle lens, and reduced with a telephoto compared to the 50mm used in the comparison.

As a guide diffraction with a 300 mm prime does not become an overall resolution issue until about 1 stop smaller than with a 24 mm.

Interesting remarks. I could not relate to laws of optics I am aware of. Can you help?

I would know these 'laws':

in the diffraction limited regime of a lens (say f/16 f/22 ...) the Airy blur function on the sensor would depend just on this aperture ratio number, not on focal length.

;--) because of the magnification of the object with the long lens I can discern more details in the object. So I may be more oblivious to the blur. That would be more a psychological effect rather than optical.

on focus closer than infinity, the effective aperture ratio would change with the aperture diameter constant but the distance to the image plane increased to

f -->  f * ( 1 + rr )

where rr is the reduction ratio from object to image. So if rr is kept the same in the image (aunt Martha always filling the frame) the diffraction effect is the same irrespective of focal length.

for a long focal length lens with a tele design, there is an exit pupil effect. On close focus the exit  pupil acts a reduced aperture diameter producing a greater depth of field and greater diffraction blur diameter on the sensor.

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