Using F-stop

• The F-Stop (Field Stop) controls the diameter of the illuminated region on the part.
• Because it is conjugate to the optical profiler object and image planes, the field stop can also be used as a focus aid.
• It is especially useful for focusing on smooth, featureless parts.
• When the field stop is pulled closed and the system is near focus, there are two images of the iris on the Live Display.
  • One image from the reference mirror of the interference objective is always in focus.
  • The second, fainter image is from the part.
  • This image will go in and out of focus as the z-stage is moved, but it will not be visible if the system is far from focus.
  • When the two images of the field stop overlap exactly, the system is in focus and fringes should be visible.
• The sensitivity of this focusing technique increases with objective magnification.