… for Random Weirdness

Tip #779: Why a Lens is Worse at f/22 than f/8

Larry Jordan – LarryJordan.com

The smaller the aperture, the greater the diffraction.

Lens aperture is determined by the iris setting.

Topic $TipTopic

This article first appeared in PetaPixel.com. This is an excerpt.

It’s common knowledge that most lenses are at their best (i.e. sharpest) between f/5.6 and f/8, depending on the lens. But why? The reason a lens is softer at f/22 than it is at f/8 is due to a phenomenon called diffraction.

Two interesting points worth highlighting are:

  • Lenses get sharper as you stop down because stopping down reduces aberration, even while it increases diffraction.
  • It’s only when the “blurry points” caused by diffraction become bigger than an individual pixel that you’ll begin to see the effect in your images.

This has two consequences that are actually noticeable in the real world:

  • All other things being equal, a higher-resolution sensor will show the effects of diffraction sooner, because the individual pixels are smaller.
  • A really well-corrected lens will begin showing the negative effects of diffraction earlier.

EXTRA CREDIT

The link at the top includes more details and a video illustrating diffraction in real life.


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