∞ generated and posted on 2016.01.14 ∞
Description of the differences in shading between what one is observing in comparison with that of the local or more-global background.
The greater the shading difference then the easier it is distinguish one specimen from background and therefore successfully view the specimen. An extreme of contrast is black versus white. However, note that it does not matter which is black versus white, specimen versus background, since a light-colored specimen against a dark back background (e.g., darkfield microscopy) is as easy to see as a dark-colored specimen against a light-colored background (e.g., brightfield microscopy).
Contrast is a hugely important component of microscopy and numerous "tricks" exist for increasing contrast including closing the iris diaphragm as much as one can get away with (light-wise), staining specimens, and employing various optical techniques (phase contrast microscopy, etc.).