The Greenough system designed by Horatio S. Greenough utilizes inclined twin body tubes to produce a zero effect. Both body tube comprises of a separate but similar lens system, each having its own ocular and objective. The major advantage of this constructive design is the high numerical apertures that can be obtained mainly because the objectives are similarly designed to a classical compound microscope. In addition, the lower portions of the body tube which contains its objectives, is in direct view of the object plane. The other end of the tube projects a pair of images through standard binocular eyepiece. The two system differs in the slightly different viewing angle at which each image is projected onto the retina. This convergence angle in turn, typically ranging from 10 to 12 degrees in modern designs, causes the left eye to view the object from the left side while causing the right eye to view the same object from a slightly different perspective on the right side.

A system of mirrors and/or prisms allows the magnified view of the image received from the objective to be correctly de-rotated and inverted to its original state. The body tubes provide a straight line-of-sight in some constructive designs while others feature additional prisms to allow movable inclination of the eyepiece, providing the microscopist a more natural viewing position. The image-forming light rays pass through the center of the body tube, forming a symmetrical quality of an image, as in most cases with a compound microscope. Furthermore, corrections for optical aberration is less difficult as compared with common main objective designs. This is because the lenses are smaller, symmetrical, and do not rely heavily on light rays that passes through the objectives.

However, a distortion caused by the oblique separation of each body tube from a common axis can be noticed. This distortion is called the Keystone effect; it causes a projection of an image wherein the area on one side is slightly larger than that of the other, producing a tapered, or wedge shape projection of a plain rectangle. This is due to the fact that the intermediate images produced by each body tube, with respect to the specimen plane, are inclined at some degree, both tilted relative to each other. So, only the regions in the center are in simultaneous focus at identical magnifications. This results in the peripheral portions of the viewing field to be slightly over focused or less focused. Although the human eye can compensate for this effect and is often not noticeable, prolonged observation periods can cause fatigue and eyestrain to the microscopist. These changes in magnification and focus across the field of view might be noticed in a photograph or video image produced through one side of the instrument, especially if the object is two-dimensional and rectilinear. This can easily be compensated by tilting either the specimen or one of the beam paths so that the microscope optical axis is perpendicular to the lateral specimen plane. Another solution to overcome this phenomenon is to tip the specimen or the microscope at some degrees and negate the convergence.