What is the sharpest Aperture On A Lens?

[srwatters]

Ask the lens designer. There are lenses specifically designed to be shot wide open. Others will benefit from being stopped down a tad. In general terms I think the following is true: The images made with consumer grade lenses will be better if shot at least 1 to 2 stops smaller than their widest aperture. This may vary with the focal length, especially with wide range or ultra wide range zooms.

For pro lenses, the only way to know is real-world testing or talking to the lens design team. Just as an example, the venerable Nikkor 200mm f/2.0 prime is specifically designed to be shot wide open or 1/3 stop off the bottom. And it does so with amazing results IMHO.

[crazeazn]

Look at the MTFs, they will tell when its sharp. As Scott said, there are few primes where even 1/3 or 1/2 stop will clean things up quickly.