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For an optical system working at infinity, calculates the minimum distance of the object and the maximum defocus possible as functions of the focal length, the clear aperture 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 depth of field is defined there as the distance between the limit positions of the object plane for which the spot size in the fixed image plane is equal to the maximum authorized diameter. Accordingly, the maximum defocus is defined as the distance between the limit positions of the image plane for which the spot size is equal to the maximum authorized diameter.