What Is Sharpness?
Here’s a diptych of two faces. It's called “Angry Man/Neutral Woman,” and was created and copyrighted by Aude Oliva of M.I.T and Philippe G. Schyns of the University of Glasgow, in 1997. It is used here with permission.
If you have relatively normal vision (corrected is okay), and are viewing this on a typical computer monitor screen or a printout, you should see an angry-looking man on the left and a calm or neutral-looking woman on the right. If you back away from your computer screen or printout, however, at a certain point you should see the faces switch places. The angry man will be on the right and the neutral woman on the left.
There’s more to this phenomenon than mere optical illusion. It has to do with the way contrast affects sharpness and the manner in which the human eye and brain perceive the world around us. There's a lesson here in how to evaluate cameras, lenses and other equipment.
Manufacturers of such gear often provide some sort of specification qualifying image resolution. It might be the number of megapixels (millions of picture elements), a count of lines (e.g., 1080), dots per inch or line-pairs per millimeter. These are all useful measures of resolution. Unfortunately (or fortunately, depending on your point of view), we’re humans, and humans tend to perceive sharpness rather than resolution.
They are most definitely not the same thing. There are some disagreements on exactly what constitutes sharpness, but the most common answer is that it is proportional to the square of the area under a curve that plots contrast ratio against resolution; the second most common is that it is proportional to the area, not its square. Either way, contrast plays a major role.
Here’s another image. This one is called a contrast-sensitivity grating. Fineness of resolution increases from left to right. Contrast ratio increases from bottom to top.
Manufacturers once routinely offered modulation-transfer function (MTF) graphs (how much contrast gets through a product at different resolutions) with their products. Today, it's a lot harder to find out about MTF, but it's worth trying. Remember: resolution numbers don't tell the whole story of sharpness, and the way in which specs are reported are subjective, at best.