The design feature of a Common Main Objective microscope (CMO) centers on the refracting action of a single, large diameter objective, shared by two separate channels of lens system. The design system is also know as a parallel microscope; having the two separate channels operating parallel to each other. This design feature guarantees that the convergence of light projected towards the left and right channels from the central objective coincides with the focal point in the specimen plane. The arrangement ensures that the left and right images viewed by the microscopist’s eyes has little or no convergence, thus, there is no inherent tilt of the image at the eyepiece focal plane.

Due to the absence of convergence at the specimen, which is usually 10-12 degrees, the brain is not able to correctly interpret three-dimensional images as supposed to the usual. This leads to a unique anomaly that is specific to CMO stereomicroscopes. The object viewed with a CMO microscope would appear as if its central portion is slightly elevated; a flat coin would appear convex. This property is called perspective distortion; it would cause much concern for an observation unless you are trying to identify the flatness and height of a certain specimen. Furthermore, specimens which are rounded in shapes, do not display significant amount of perspective distortion. This perspective distortion is also called doming or sometimes called the globular effect; this is a result of the combination of the keystone effect, and pincushion distortion. This can sometimes be noticed among less expensive microscopes but modern-day manufacturers of stereomicroscopes have already eliminated this problem regarding their instruments.

Other distortions suffered by CMO stereomicroscopes are off-axis aberrations; coma, astigmatism, and lateral chromatic aberration that appears in the center of each image as projected. This is due to the fact that the light rays received from the separate channels are from an off-center region of the main objective and not directly from the center. These effects however, are only noticed when viewing the specimen on one of the two channels especially when taking digital images of the specimen; no distortions can be noticed when viewed by both eyes though. This can be corrected by offsetting the central main objective, positioning it on the central axis of one of either channels. There are also other designs that provides a means to replace the main objective with infinity-corrected objective for viewing and photographing specimens. These corrections would somehow be costly due to the large size and volumes of lens glasses used in manufacturing these large common main objectives. The most practical advantage of this design feature, as compared to the Greenough princilple, is its versatility and expandability with the introduction of an infinity optical system; it can effortlessly fitted with accessories such as illuminators, beamsplitters, photo or digital video recorders and other corrective lenses. This is possible because of the collimated pathway for the light rays that passes through the lens system—there is no angle which means little or no specific light corrections.