Do you have enough room under your microscope?
Focal length and working distance are critical. In order to accommodate your instruments, you need enough room under your microscope. Focal length for surgical microscopes comes in two types: fixed focus and variable focus.
Fixed focal length
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.
Variable focal length
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. Plus, 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 in the 300 to 450 mm range.
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. Usually, it is imprinted with f=xxx on the lens’ bezel.
The working distance can then be determined. For example, if the front lens has a focal length of f=200, the object will not focus at less than 200 mm working distance. In addition, using a 300 mm instrument would not be advised.
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