FOCUSING

A compound microscope is focused by moving the entire assembly, including both the eyepiece and the objective, up and down. This must be done with a precision mechanism because the depth of field (the difference between the shortest and the greatest distance from the objective at which an object is in good focus) is exceedingly small. In general, the shorter the focal length of the objective, the smaller is the depth of field, and the more critical is the focusing. High-magnification objectives have depths of field on the order of 2 μm (2 × 10^-6 m) or even less.
If the eyepiece is moved up and down in the microscope tube assembly while the objective remains in a fixed position, the magnification varies. However, microscopes usually are designed to provide the best image quality for a specific eyepiece-to-objective separation, such as 16 cm (approximately 6.3 in).
If a bright enough lamp is used to illuminate the specimen under examination, and especially if the specimen is transparent or translucent so that it can be lit from behind, the eyepiece can be removed from the microscope and a decent image can be projected onto a screen on the ceiling of the room. A diagonal mirror can reflect this image to a screen mounted on a wall. This technique works best for objectives having long focal lengths, and hence low magnification factors.