Different types of stereo microscopes
Stereo microscopes (sometimes referred to as dissecting microscopes) are relatively low power microscopes typically used to view whole samples (components on a printed circuit board), or where any sort of sample manipulation is required, such as dissection or watch making. Essentially a stereo microscope is just two microscopes (with separate optical paths) which focus on the same point but from slightly different angles. This creates a 3-dimensional 'stereo' image of the object, exactly the same as when you use both of your eyes!
A 'stereo' image is a very natural image, with depth perception so is ideal for applications where hand-eye coordination is required. Stereo microscopes also have a considerably larger working distance than compound microscopes, allowing manipulation tasks, such as dissection, to be performed with relative ease. Magnifications used can range from as little as 2x up to 300x, but typically might be between 20x and 80x, depending on what you are looking at, of course. Stereo microscope can have a fixed magnification (or several selectable fixed magnifications) or a zoom magnification system. Both systems have advantages and disadvantages, but it depends on what your requirements are and of course, your budget.
Greenough and CMO (Common Main Objective) stereo microscope designs
Your brain and eyes function together to produce "stereoscopic vision", which provides spatial, three-dimensional images of the objects surrounding us. The average human eyes are approximately 6.5cm apart, each with a slightly different viewpoint of the world from the other. When an image is transmitted to the brain, the resulting image is fused together, retaining a high degree of depth perception. Stereo microscopes take advantage of this depth perception ability by transmitting twin images that are inclined (usually between 10 and 12 degrees) to yield a true stereoscopic effect.
Modern stereo microscopes can be divided into two basic families. Greenough stereo microscopes (named after the inventor) utilise twin body tubes that are inclined to produce the stereo effect. Whereas CMO stereo microscope use a single Common Main Objective (CMO) that is shared between a pair of eyepiece tubes and lens systems.
In general CMO stereo microscopes have a greater light-gathering power than the Greenough-type design and are often more highly corrected for optical aberration, though in most circumstances, the choice between Greenough or CMO stereo microscopes is usually based on the application, rather than whether one design is superior to the other.
Greenough stereo microscopes are typically employed for "workhorse" applications, such as printed circuit board inspection, dissecting biological specimens, or similar routine tasks. These microscopes are relatively small, inexpensive, very rugged, simple to use, and easy to maintain.
CMO stereo microscopes are generally used for more complex applications requiring high resolution with advanced optical and illumination accessories. The wide spectrum of accessories available for these microscopes lends to their strength in these areas.
However, often a major consideration is cost. CMO stereo microscopes can cost several times more than a Greenough stereo microscope, so this naturally will strongly influence buying decisions.