F-number

The F-Number of a lens is defined by the ratio between the EFL and the diameter of the entrance pupil. The ImageMaster® series instruments use the ISO-517 method to accomplish the measurement: First the EFL is measured according to the procedure described before. Then the sample is turned around with the entrance pupil facing towards the detector head. The diameter of the entrance pupil is measured by first focusing on the left, then on the right edge of the pupil. The travelling distance of the image field stage represents the diameter of the entrance pupil. By choosing the proper head lens of the detector head even pupils positioned far inside of the system under test can be measured.