How do you choose the sharpest aperture for a lens, and when does stopping down reduce image quality?
Asked 6/9/2011
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I’ve read that very small apertures (for example f/16, f/18, f/22 and smaller) can reduce image quality. Is that true, and if so why?
If I’m shooting from a tripod and I’m free to use any shutter speed or ISO, how do I choose the aperture that gives the best image quality? Does the “best” aperture depend on the lens, or more on the camera sensor? Also, how does this relate to depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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The issue you are talking about is diffraction. It is less a lens issue (all lenses will cause diffraction) and more a sensor issue.
As light enters a small aperture, the light waves can diffract and interfere with each other. This can result in the airy disk that any given light wave casts on the sensor being larger than the pixel size of the sensor, and so there is a resultant loss of quality.
However, in real world situations it is debatable how much loss of quality is actually visible in normal viewing. Post-processing and printing can hide a multitude of sins.
In the situation you describe in your question (which is essentially a landscape shot) I would probably set f/16 as a good compromise between diffraction and DoF, and make use of hyperfocal distance to ensure as much front to back sharpness as possible.
I was going to link to Cambridge in Color but gerikson has beat me to it: it is a good article, if a little technical.
EDIT: Another aspect of this occurs to me. You mentioned 'the highest-quality aperture' for a lens, and lenses do indeed have a 'sweet spot' which is usually 1-2 stops off wide-open. However, this gives DoF issues in certain situations, i.e. landscapes.
Originally by user3205. Source · Licensed CC BY-SA 4.0
user3205
15y ago
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Yes. Stopping down too far can reduce sharpness because of diffraction. As the aperture gets smaller, light spreads more, creating a larger airy disk on the sensor and lowering fine-detail sharpness.
This is not just a lens issue; it also depends on sensor size and pixel density. The aperture where diffraction starts to noticeably reduce per-pixel sharpness is often called the diffraction-limited aperture. Higher-density sensors and smaller sensors tend to show it sooner.
Rules of thumb from the answers: around f/11 on many APS-C DSLRs, and even around f/5.6 on small-sensor cameras with dense pixels. In real-world viewing, the loss may or may not be obvious.
So the “highest quality” aperture is not always the smallest aperture. If you only want maximum peak sharpness, it is usually somewhere before heavy diffraction sets in. But if you need more depth of field, stopping down further can still improve overall image sharpness across the scene, even if the absolute sharpness at the focus plane drops.
For landscape-type use, f/16 is often a practical compromise, especially when combined with hyperfocal focusing.
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