The compound microscope consists of two optical components (thus the term compound): the objective lens system, which has a very short focal distance and is placed very close to the object; and the ocular or eyepiece system, which has a longer focal length, lower magnification; and which further magnifies and projects the image onto the retina of the eye. The objective lens projects a real image (the intermediate image) of the object up in the body of the microscope onto the objective conjugate image plane or primary image plane. The distance from the back focal plane of the objective to the primary image plane within the microscope body is referred to as the optical tube length. Depending on the type of microscope and eyepiece, the primary image plane may be either within the microscope body itself or within the eyepiece (see Infinity corrected optics). The objective lens forms a real image in the microscope body that acts as the object for the ocular lens. The real image becomes the object for the eye itself and is projected onto the retina. The ocular in turn acts as a simple magnifier to form a large virtual image at the distance of most distinct vision (25 cm) from the eye.

The objective and ocular lenses act in series. Thus, the total magnification in a compound microscope is a function of the objective magnification multiplied by the eyepiece magnification.

The Compound Microscope
The advantage of having a compound lens system as opposed to a simple magnifier is that much higher magnifications can be achieved with two lenses rather than one. To achieve high magnification with single-lens magnifiers (a magnifying lens, for example) the lens itself must be placed close to the eye to increase the subtending angle, and thus the size, of the projected image on the retina The eye then focuses on the large virtual image formed by the positive lens magnifier. Modern magnifying lenses, as well as Leeuwenhoek's first magnifier, worked in just such a manner. A problem arises when very high magnifications are required because lenses cannot be placed comfortably close enough to the eye to result in a large subtending object-eye angle. The single-lens, double-convex magnifier of van Leeuwenhoek achieved at most 275x magnification.
Light from the Eyepiece takes two separate paths depending on its origin. If it is sample light (the real image of the sample) the rays emerge from the eyepiece nearly parallel. At their most focused point they form a circle of light approximately 3mm in diameter. This circle (or the circle of light from any lens) is called the Exit Pupil. The distance from the front glass surface to the exit pupil is called the Eye Relief. This is usually 15-17mm, but can be further in High-Eyepoint oculars.

A compound microscope can be viewed as having two simultaneous, but mutually exclusive sets of conjugate planes. One set, the Sample Image Planes, consist of the following: A: Field Iris, B: Sample Plane, C: Intermediate Image, and D: Retina.

The other important set, the Sample Illuminating Planes, consist of E: Light source (Filament), F: Condenser Iris, G: Objective back focal plane, H: Exit Pupil.