Patent ID: 7453581

Claim:
A method of determining the focus of an optical system, which method comprises the steps of: arranging the optical system between an illumination system and a radiation-sensitive detection system; arranging a test object between the illumination system and the optical system; imaging the test object by means of the optical system on the detection system, and establishing the position of the focal plane from the modulation depth of the test object image, characterized in that the step of arranging a test object comprises arranging a first test object between the illumination system and the expected focal plane of the optical system and a second test object between this plane and the optical system, whereby the first and second test objects are arranged at different locations with respect to the optical axis of the optical system, and in that the step of establishing focus comprises determining the modulation depth of each of the two test object images and subtracting the obtained modulation depth values from each other to obtain a differnce value, which represents the position of focus with respect to the positions of the two test objects.