Diffraction limiting

Started 2 months ago | Discussions thread
Great Bustard Forum Pro • Posts: 44,678
Diffraction...
6

Ad12 wrote:

Hello!

On imaging resources, I noted my new XF 18-55 says diffraction limiting starts at about f16. My old lens for my Canon the sigma 17-50 they say it begins at f8. I must admit, the lens was usually super sharp, but did definitely soften up after about f10 in my experience quite a lot.

I always mistakenly thought diffraction limiting was kind of similar for lenses at a certain sensor size. this suggests not.

So is it the lens that really dictates this? Not interested in pixel by pixel worrying, just curious!

...goes like this:

  • Diffraction softening affects *all* systems equally at the same DOF.
  • Diffraction exists right from wide open and increases as you stop down.
  • Lens aberrations exist right from wide open and lessen as you stop down.
  • The relative aperture at which diffraction softening dominates over lessening lens aberrations is often called "the diffraction limited aperture".
  • The "diffraction limited aperture" is only moderately a function of pixel size, where smaller pixels, all else equal, will achieve the "diffraction limited aperture" at a lower f-number. However, more smaller pixels allows you to see the effects of diffraction earlier, because the effect of diffraction is hidden in the *greater* blur of fewer larger pixels. Thus, more smaller pixels (all else equal) will *always* resolve more, stop-for-stop, than fewer larger pixels.
  • The "diffraction limited aperture" is, however, strongly a function of the particular lens, because the lens aberrations vary tremendously from lens to lens whereas diffraction is the same.
  • Typically, the "diffraction limited aperture" will be a stop later at the edges than the center, because the edges have more aberrations than the center.
  • Diffraction is often the least of your problems when stopping down "too far" -- stopping down will result in less light on the sensor (for a given scene and exposure time) resulting in a more noisy photo or may result in excessive motion blur (for a given scene and exposure), either of which may well be more damaging to the IQ of the photo than diffraction itself.

Thus, the "advice" that was given:

I noted my new XF 18-55 says diffraction limiting starts at about f16. My old lens for my Canon the sigma 17-50 they say it begins at f8.

is just plain wrong.

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