A simple microscope is a microscope that uses only one lens for magnification, and is the original light microscope. Van Leeuwenhoek’s microscopes consisted of a single, small, convex lens mounted on a plate with a mechanism to hold the material to be examined (the sample or specimen). Demonstrations by British microscopist Brian J. Ford have produced surprisingly detailed images from such basic instruments. The use of a single, convex lens to magnify objects for viewing is found today only in the magnifying glass, the hand-lens, and the loupe. The compound microscope uses a set of many lenses in order to maximize magnification. In its simplest form - as used by Robert Hooke, for example - the compound microscope would have a single glass lens of short focal length for the objective, and another single glass lens for the eyepiece or ocular. Modern microscopes of this kind are usually more complex, with multiple lens components in both objective and eyepiece assemblies. In modern microscopes the mirror is replaced by a lamp unit providing stable, controllable illumination.

Compound optical microscopes are typically used to examine a smear, a squash preparation, or a thinly sectioned slice of some material. With a few exceptions, they utilize light passing through the sample from below and special techniques are usually necessary to illuminate the sample to increase the contrast in the image to useful levels (see contrast methods). Most compound microscopes use Köhler illumination (named after its inventor August Köhler) to achieve maximum resolution power at higher magnifications. However, at low power they may also be used to examine small living animals and plants and even at high power, microscopic living speciments such as protozoa, rotifers, and bacteria may be examined. Typically, on a standard compound optical microscope, there are three objective lenses: a scanning lens (5×), low power lens (10×) or sometimes medium power lens (20x), and high power lens (40×). Advanced microscopes often have a fourth objective lens, called an oil immersion lens. To use this lens, a drop of immersion oil is placed on top of the cover slip, and the lens is very carefully lowered until the front objective element is immersed in the oil film. Such immersion lenses are designed so that that the refractive index of the oil and of the cover slip are closely matched so that the light is transmitted from the specimen to the outer face of the objective lens with minimal refraction. An oil immersion lens usually has a power of 100×. Older microscopes were sometimes equipped with water immersion lenses which had a higher performance than non-immersion lenses but which were still significantly inferior in performance to oil immersion lenses.

Stereo microscopes, also called dissecting microscopes, are really two compound microscopes which focus on the same point from slightly different angles. This allows the specimen to be viewed in three dimensions. As opposed to compound microscopes, the image is upright and laterally correct (not upside down and backwards). Stereo microscopes are relatively low power compared with compound microscopes, usually below 100x. They can have a single fixed magnification, several discrete magnifications, or a zoom magnification system. Working distance is much longer than with a typical compound microscope as well, allowing work to be done on the specimen while it is being observed through the microscope (hence the name “dissecting microscope”). Many stereo microscopes are modular in design allowing a variety of stands, eyepieces, objectives, and lighting techniques to be implemented depending on the intended use.