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Focimeter

Last updated 6 October 2026

Practice MCQs (29)
  • The focimeter (lensmeter) measures the back vertex power of a spectacle or contact lens: sphere, cylinder, axis, prism and add
  • It also marks the optical centre and the cylinder axis on the lens

Principle

A focimeter. Top: an illuminated target lit by green light sits at the focal point of the collimating lens, so light leaves it parallel and the target is seen sharp in the viewing telescope. Bottom: a minus test lens is placed after the collimating lens; the target has been moved along its scale (from −20 to +20) until the light leaving the test lens is parallel again.
Without a lens the target sits at the collimating lens's focal point; a test lens is measured by how far the target must move to restore parallel light
  • An illuminated target moves along the…
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