[Intermediate] Lens sharpness vs. F/STOP

from http://www.kenrockwell.com

Lens sharpness varies wildly with aperture. let's look at a 50mm f/1.4 AI for this example.
Any of the other f/1.4 lenses would do the same thing.


Nikon 50mm f/1.4 AI


1.4 AI at f/1.4 ------------- 1.4 AI at f/2 ------------- 1.4 AI at f/5.6 ------------- 1.4 AI at f/16


SUMMARY

Your choice of aperture is far more important than your choice of lens!

Lens performance varies wildly from one f/stop to the next.

It's easy to make mistakes when comparing lenses. If you compare at different f/stops you're not comparing lenses. Many laypeople make the mistake of comparing two lenses at different times of day, or different f/stops. Any time you change anything in your test, you're no longer comparing two lenses.

Most lenses are their sharpest from two stops below maximum to about f/8. Avoid apertures smaller than f/8 because diffraction softens them.

f/1.4 lenses are very sharp from f/2.8 onwards, while slower zooms may have a much smaller range of optimum apertures. Luckily good zooms are quite sharp wide open, unlike most f/1.4 lenses.