Focusing:

The compound microscope is focused by moving the whole assembly, involving both the eyepiece and the objective, up and down. This should be completed with an exactness mechanism as the depth of field (i.e., the difference among the shortest and the greatest distance from the objective at which an object is in fine focus) is very small. In common, the short the focal length of the objective, the smaller is the depth of field, and the more critical is the focusing. The high-magnification objective consists of depths of field on the order of 2 µm (2 x 10^-6 m) or even less.

When the eyepiece is moved up and down in the microscope tube assembly whereas the objective stays in a fixed location, the magnification differs. Though, microscopes generally are designed to give the best image quality for a particular eyepiece-to-objective separation, like 16 cm (around 6.3 in).

When a bright sufficient lamp is used to light up the specimen under inspection, and particularly when the specimen is transparent or translucent therefore it can be lit from at the back, the eyepiece can be eliminated from the microscope and a decent image can be projected onto a screen on the ceiling of the room. The diagonal mirror can reflect this image to a screen build up on a wall. This method works greatest for objectives containing long focal lengths, and therefore low magnification factors.