Fixed focal length convergent lenses, usually f=175 or f=200, are used for ophthalmic procedures. Some operating microscopes for ENT procedures may also use a fixed front lens.

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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).

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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.

The focal length is variable for neurosurgical, spine, and ENT procedures. 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.

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Focal length and working distance are critical. You need enough space under your microscope to accommodate your instruments. Surgical microscope focal lengths vary in two types: fixed focus and variable focus.

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.

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