Radtech1 wrote in post #2106933
What I am interested in is links, if you know of any, to empirical tests/data that show where lenses are sharpest.
Rather than provide links to online lens tests, I would rather address the underlying basis for that rule of thumb. That way, instead of trying to verify a rule, you'll know how to make your own rules.
Every lens has its own sweet spot. As you stop down, the fuzziness caused by diffraction increases. Most lenses show visible diffraction effects at the smallest apertures.
Most lenses also reveal their inherent optical faults when they are used wide open. As you stop down, the effects of these faults diminish.
The sweet spot is that point where both lens faults and diffraction effects are jointly minimized.
Some lenses perform best wide open. I have a Zeiss Jena Sonnar that is better at f/2.8 than at f/11. And I expect my 70-200/4L is nearly as good at f/4 as it is at f/8. I have a large-format Schneider Super Angulon that performs optimally at f/22-f/32, but at f/45 diffraction effects are clearly visible. Thus, there's no way to have a general rule of thumb.
The closest you can come is to say that most lenses will perform best at the smallest aperture that keeps the effects of diffraction below the threshold of acceptability. For some of my medium-format lenses, that's f/11 or even f/16. But some lenses are so well-optimized for wide-open usage that there is no effective improvement from stopping down except as a result of increased depth off field.
Just take some pictures of a detailed target over a range of aperture settings and look at the images in actual pixels on your computer monitor. That will tell you what the optimum aperture is for that lens. But remember that the optimum aperture isn't useful if it forces you to use a shutter speed that allows motion blur, or if it lacks sufficient depth of field.
Rick "who thinks other influences on aperture are more important" Denney
