Understanding Focal Length (Working Distance)

Do you have enough room under your microscope?

bumping-head
          Not Enough Room?

Because you need enough room under your microscope to accommodate the lengths of your instruments and you want want to consider how much anatomy you want in your Field of View. 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 focal length 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 this focal length will be in focus. Any object outside the focal length will never be in focus. The working distance from the surgical site (the objective) can range from 200 to 450 mm. So if you use an instrument that is 300 mm long, you may not want the microscope focused 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).

working-distance-eyes

  • Convergent (double convex) lenses are used. This is the “Front Lens” of the microscope 
  • It is usually imprinted with the focal length (f=) 
  • Focal Length is also known as the Working Distance which is the distance between the front lens and the object plane
  • A fixed focal length means the object will be in focus from 0 mm to f=xxx.
  • The working distance can be approximated: for example if the front lens is f=200, the object will stay in focus if the front lens is 0 mm from the object or 200 mm from the object
  • This diagram shows the “rays” from the object plane being converted by a fixed focal length convergent lens and then shaped by a series of prisms for binocular viewing.