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Focal Length Measurement System


You want to know the exact focal length of a lens because if you pay for a 400mm lens, it's nice to know you have a 400mm lens, not a 370mm lens. In fact most lens makers will tell you that the focal length marked on a lens is +/- 5%. That means your 400mm could be as short as 380mm and still be within spec.


lens is surrounded by air or vacuum (refractive index 1.0), the focal length is  the distance from the principal point to the focal point of a lens.




Measurement parameter

Focal Length

F- Number


Edge Thickness

Centre Thickness