For many days, I was looking for my tripod. Finally found it couple days ago. Perfect timing for the Vivitar 28mm f2.0 sharpness test. This is the first "serious" sharpness test I have done for a lens, and I learned that I didn't have to do the previous vignetting test at all, as the sharpness and vignetting test can be done at the same time. Oh well, live and learn. Better planning next time.
First thing first. This is not a scientific test. Please take it with a grain of salt. Also, copy variance exists, so this lens may perform better or worse than yours. I take no responsibility and no guarantee of the results will resemble your lens.
Also, the test images were shot as RAW + Jpeg (L). I used the camera generated jpeg files instead of converting the images from RAW. I think this equalizes the results across all of the images. The RAW file, when converted, is slightly sharper with better quality. If you need the RAW file, leave a comment and I will send you a copy.
Finally, doing test like this is tedious and boring. It takes up a lot of time, but I hope the results are usable for you if you are thinking of buying one of these lenses used.
Lens -- Vivitar 28mm f2.0 Ai-S mount, serial #28409972.
Lens Condition -- Excellent physically and cosmetically. No sign of abuse.
Adapter -- Cheap Focus Assist Ai to EOS adapter. Focus Assist feature not used
Camera -- Canon 5D with EE-S screen to aid manual focusing. Mirror lockup and timer used
Tripod -- Volbon Carmagne 630 carbon fiber tripod
Tripod Head -- Acratech Ultimate Ball Head
Test Target -- Wall
Summary of test --It's a nice little lens. Quite sharp even wide open at the center. Moderately heavy vignetting until f4. As expected, far corners performance isn't very good but improves a lot by f8. You can see the barrel distortion in the full picture. Sorry I don't have software or instrument to test the amount of distortion, but it's visible.
Full Image. A little under exposed -- Larger Picture.
Wide Open at f2. 100% Crops. Larger Picture.
Half a stop down at f2.5. 100% Crops. Larger Picture.
One stop down at f2.8. 100% Crops. Larger Picture.
f3.5. 100% Crops. Larger Picture.
Two stops down at f4.0. 100% Crops. Larger Picture.
f4.5. 100% Crops. Larger Picture.
Three stops down at f5.6. 100% Crops. Larger Picture.
f6.3. 100% Crops. Larger Picture.
Four stops down at f8.0. 100% Crops. Larger Picture.
f9.0. 100% Crops. Larger Picture.
Five stops down at f11.0. 100% Crops. Larger Picture.
Minimum Aperture at f16. 100% Crops. Larger Picture.