focus shift

Focus shift is a term that refers to the phenomenon in which a lens’ focus plane (the plane where focus is optimal) moves backwards or forwards as the lens is stopped down. Focus shift is an optical flaw or aberration (it does not happen to all lenses), and you should feel free to study JAPB’s detailed article on focus shift.

Focus shift (caused by stopping down a lens) should not be mixed up with the shifting of focal planes caused by a change of focal length in zoom lenses. That phenomenon is known as Zoom-induced focus shift or zoom focus shift.

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