The microscope is an instrument designed to make fine details visible. The image of an object can be magnified when viewed through a simple lens. By combining a number of lenses in the correct manner, a microscope can be produced that will yield very high magnification values.

The optical components contained within modern microscopes are mounted on a stable, ergonomically designed base that allows rapid exchange, precision centering, and careful alignment between those assemblies that are optically interdependent. Together, the optical and mechanical components of the microscope, including the mounted specimen on a glass micro slide and cover slip, form an optical train with a central axis that traverses the microscope base and stand.

One of the most critical aspects in optical microscopy is to ensure the specimen is illuminated with light that is bright, glare-free, and evenly dispersed in the field of view. Modern microscopes usually have an integral light source that can be controlled to a relatively high degree. The most common source for today's microscopes is an incandescent tungsten-halogen bulb positioned in a reflective housing that projects light through the collector lens and into the sub stage condenser. Other sources include arc-discharge lamps, light emitting diodes and lasers.

Regardless of the imaging mode utilized in optical microscopy, image brightness is governed by the light-gathering power of the objective, which is a function of numerical aperture. Just as brightness of the microscope source illumination is determined by the square of the condenser working numerical aperture, brightness of the specimen image is proportional to the square of the objective numerical aperture.

Microscope objectives are the most important components of an optical microscope because they determine the quality of images that the microscope is capable of producing. There is a wide range of objective designs available that feature excellent optical performance and provide for the elimination of most optical aberrations.

In the optical microscope, image formation occurs at the intermediate image plane through interference between direct light that has passed through the specimen unaltered and light diffracted by minute features present in the specimen. The image produced by an objective lens is conjugate with the specimen, meaning that each image point at the intermediate plane is geometrically related to a corresponding point in the specimen.

Departures from the idealized conditions of Gaussian optics are known as optical aberrations. Microscope optical trains typically suffered from as many as five common aberrations: spherical, chromatic, curvature of field, comatic, and astigmatic. Geometrical distortion is another artifact often encountered in the zoom lens systems found in stereoscopic microscopes.

The modulation transfer function of a lens, microscope objective, or other optical system is a measurement of its ability to transfer contrast at a particular resolution level from the object (or specimen) to the image. Computation of the modulation transfer function is a mechanism that is often utilized by optical manufacturers to incorporate resolution and contrast data into a single specification.