I'm not talking about large aperture (f/1.8, f/2.8...), rather, about small apertures (f/18, f/20, f/23...). I read somewhere (actually I think it was on this site, but I don't exactly remember which post/comment it was) that lens start to lose their quality at small apertures, like f/16 and smaller. Is this true?
Assuming the following situation:
- you have a tripod
- you have as much light as you need
- you don't care about the shutter speed
- you don't care about ISO
- you don't care about the motion blur, or its lack, thereof
Therefore, all you care about is choosing the highest quality aperture. What value will that be and how is it different among slow and fast lenses?
Answer
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.
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