Originally posted by slipchuck I was wondering if 2.8 is soft throughout the range, is it worth spending the extra for a 2.8? wouldn't a 3.5-4.5 that is sharp wide open (and could be less money) make more sense?
Others have already correctly answered good part of your question.
I would like to add one more thing: every lens is sharper at mid-range apertures. Thus, an f/4 or f/5.6 lens will still be sharper at smaller apertures, say f/8 or f/11.
Important thing to remember is that no lens in the world can have equal sharpness at all apertures. While measuring lens resolution reviewers often provide relative comparison (the same lens at different apertures) pointing out that a lens is "soft" wide open and sharper stopped-down. No surprise here, that is always true for any lens and should not be tested at all.
Here is the principle behind this: at wide apertures lens aberrations are more pronounced (chromatic, spherical, astigmatism, field curvature...) and this effect becomes less visible with smaller apertures. But with apertures becoming too small aperture blades diffract light and image again looses sharpness. Obviously, there is a mid point where decreasing aberrations cross increasing diffraction influence, and that point is the sharpest aperture.
My tests with some 5 or 6 different lenses confirmed that ALL of them are sharpest at f/8 and f/11. Larger the maximal aperture and this interval extends down. Examples: my A50/1.4 is sharpest at f/4 to f/11; A28/2.8 is sharpest at f/5.6 to f/16; FA35/2AL is sharpest between f/4 and f/11 (and only slightly softer and f/2.8 - an excellent lens BTW!).
But there is one final and subtle detail: at large apertures depth of field is so small that there is more risk of having softness due to a focusing error than to any lens "softness" at wide apertures.
Some interesting resources:
1) Interesting articles on optics and camera lenses:
www.pinnipedia.org/optics.html
2) Famous Yoshihiko Takinami lens resolution test page -- you can clearly see how lens resolution increases and then decreases with various apertures for all lenses tested:
www.takinami.com/yoshihiko/photo/lens_test/index.html
3) WARNING: a totally geeky resource!!! Laboratory optical tests of many Pentax lenses with light paths and lens optical designs. Note that XY diagrams are not to scale -- to compare various lenses make sure to read numbers at X/Y axes (do not compare visually only). Check it out here:
pentaxstudy.bufsiz.jp