Darkfield is therefore the method whereby the sample being viewed is actually in front of a dark background and light is being angled onto the sample from the sides.

Under phase contrast conditions, the light coming through the specimen is shifted into two beams, one slightly out of phase with the other. This gets a little complicated to explain easily, but as far as equipment concerns, you need two matched items in order to get phase contrast.

One needs a phase annulus, and the matching lens objective. For instance, if you want 40x magnification phase contrast microscopy, you need a 40x phase lens, and a matched 40x phase annulus. If you want 100x phase, you need the 100x lens and the matched 100x phase annulus.

Both the techniques of darkfield and phase contrast allow nearly invisible microorganisms within the blood to be "lit up" and seen. It also clearly delineates the blood cells. This method is in contrast to the standard microscope "brightfield" conditions where light shines directly through the viewed sample, and invisible particles remain invisible.