Lens sharpness and aperture

[tomczak]

Trying to test the lens sharpness as a function of aperture, I set the camera on a tripod and shoot the scene with lots of details and textures at base ISO and f2, f2.8, f4, f5.6 and f8. The details get consistently and progressively more blurred as the aperture gets smaller (i.e. they seem the sharpest at f2, maybe f2.8, but the two are difficult to distinguish, and the softest at f8).

This is counter intuitive to me. Is it possible that the lens is optimized for f2? I was shooting JPGs, so it's possible that the level of noise reduction increased with the increasing f-stop, but why would it - the ISO didn't change...

p.s. as I was writing this question, I suddenly recalled Sean McHugh's article on diffraction and Airy Disks. There is a Java calculator there, and the results seem to be consistent with what I've seen. For a small sensor (1/1.7") it seems possible that the camera is diffraction-limited even at quite large apertures.

http://www.cambridgeincolour.com/tutori ... graphy.htm

[alain]

This is indeed possible, but without camera information this is guesswork

[tomczak]

It's Canon S95 - a 10Mp, 1/1.7" sensor camera. Sean's lens/sensor diffraction model predicting at what apertures it may become a visible nuisance (the Java calculator on the second page of the article quoted before), computes it as around f4. This is amazingly close to my informal observations. The f2.8 seems to be the sharpest, preserving low contrast texture the best; f2 is slightly less sharp (which is not caused by diffraction but rather lens optical aberrations magnified at large apertures); from f4 on the diffraction kicks in, negating the benefits that smaller apertures may have head on the lens performance).

I'm convinced it's diffraction now, and the effects are subtle but visible. I think that the this camera/lens is amazing image quality wise, especially for its size. According to the same diffraction model, a APS size sensors would not notice it until f11 or so and even more for larger pixels, which is probably why I haven't realized its importance so far.