Conjugate focus

(Opt.) See under Focus.
(Opt.) the focus for rays which have a sensible divergence, as from a near object; - so called because the positions of the object and its image are interchangeable.

See also: Conjugate, Focus

References in periodicals archive ?
In a nutshell, to test the concave secondary you place a pinhole light source at one of the secondary's foci and examine its image with a knife edge positioned at the conjugate focus.