Focal length and working distance are critical. You need enough room under your microscope to accommodate your instruments plus you want to consider how much anatomy you want in your Field of View. Surgical microscopes come in two types: fixed focus and variable focus.
For ophthalmic procedures, fixed focal length convergent lenses are used. Usually f=175 or f=200. Some operating microscopes for ENT procedures also may use a fixed front lens.
For neurosurgical, spine and ENT procedures, the focal length is variable. For example, the objective lens combination can be configured so the focal length is from f=200 to f=450. So any object within f=200 to f=450 will be in focus. Any object outside f=200 to f=450 will never be in focus. The minimum working distance from the surgical site objective is 200 mm . So if an instrument is 300 mm long you would not want the microscope at 200 mm.
Also, remember that with increased focal length (working distance) more of the objective will be in view. This is called Field of View (magnification aspect discussed later).
The “Front Lens” of the microscope is the convergent (double convex) lens. The focal length is the distance between the front lens and the object at F. It is usually imprinted with f=xxx on the lens’ bezel.
The working distance can then be approximated. For example if the front lens has a focal length of f=200, as a result the object will not focus at less than 200 mm working distance. In addition, using a 300 mm instrument would not be advised.
For information on magnification, please click HERE
For information on Haag-Streit Surgical Microscopes, please click HERE