I previously discussed how camera shake and focus accuracy can affect maximum image sharpness with the Nikon D800 (or any DSLR, for that matter). Another contributing factor to image sharpness is lens performance, especially with respect to aperture. While the primary use of aperture is to control depth of field, most lenses just aren’t quite at their sharpest when used at their extremes.
Wide-open, most lenses will have sub-optimal performance. Contrast and sharpness will be reduced, and you might also see light fall-off (corner shading). As you stop the aperture down, both sharpness and depth of field increase, and light fall-off diminishes. How much you need to stop down to get razor-sharp images is related to the quality of your lens. Some of the telephoto prime lenses, like the Nikon 300mm f/4 lens, are very sharp wide-open, especially compared to a consumer-grade counterpart, like the 70-300mm f/4.5-5.6 VR Nikkor (a good lens, but not nearly as sharp wide-open). Higher quality lenses usually require less stopping down to get optimal sharpness.
However, beyond certain apertures, images will actually start to soften due to the effect of diffraction. Diffraction softening is a physical function related to the size of the pixels on your camera sensor. The smaller the pixels, the more noticeable diffraction effects can become. Continue reading