The truth is wide-aperture primes are usually not exceptionally sharp wide open. They appear to be sharper than they really are because the in-focus subject stands out so much from everything else (assuming the subject is near the center) given the narrow depth of field wide open. But if you measure the absolute resolution numerically, a prime lens shot at f/1.4 is no sharper (and in many cases less sharp) than a top quality zoom shot at f/2.8.

Let’s look at a very good wide-aperture prime, the Canon 35mm f/1.4 “L” lens. Here are the Imatest results for fifteen Canon 35mn f1/.4 lenses measured at both f/1.4 and f/2.8 apertures. (The number on the vertical axis is the MTF 50 weighted average for the entire lens, that on the horizontal axis is MTF50 at the center point).



More lenses compared:


Small conclusion:
1.Some lenses benefit greatly from being stopped down, others very little.
2.For those lenses that do benefit, stopping down just to f/2.0 provides the majority of resolution improvement. The difference between wide open and f/2.0 is generally much greater than the difference between f/2.0 and the maximum resolution.
3. Getting the edges and corners sharp requires stopping down to at least f/4 for most wide-aperture primes, and some really need f/5.6. Stopping down to f/2.8 may maximize center sharpness but often makes only a slight difference in the corners, at least on a full-frame camera.
4. None of the lenses performed any better after f/5.6 (for the center) or f/8 for the corners. Most were clearly getting softer at f/11.

Read full article at: http://www.lensrentals.com/blog/2011/11/stop-it-down-just-a-bit