System and method for measuring the focus state of an optical instrument

A system for measuring the focus state of an optical instrument, said system comprising:

• at least one first optical element configured to form an image of an exit pupil of an objective of said optical instrument;
• at least one second optical element placed substantially where said image of said exit pupil is formed and configured to deviate at least part of the rays coming from said exit pupil so that, downstream said image of said exit pupil, first rays coming from a first portion of said exit pupil and second rays coming from a second portion of said exit pupil follow separate optical paths, said first portion and said second portion of said exit pupil being non overlapping, and are focused into two bidimensional images;
• and
• a computer configured to determine a mutual distance between said two bidimensional images as a mutual rigid lateral displacement between said bidimensional images and, based on said mutual distance, to determine a corresponding defocus of said optical instrument.