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For an optical system, calculates the image position and the maximum defocus possible as functions of the focal length, the clear aperture, the object position and the maximum diameter authorized for the image spot. The aperture is located on a principal plane (primary principal plane if in the object space or secondary principal plane if in the image space).

The maximum defocus is defined there as the distance between the limit positions of the image plane for which the spot size is equal to the maximum authorized diameter.