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

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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. Since this is a null test, math and micrometer readings don't matter--you simply aim for a null, or perfectly straight lines with a Ronchi grating.