Close-up Diopter
A magnifying filter that screws onto the front of a lens to allow closer focusing without losing light.
Formula / rule
Working distance = 1000 / diopter power (mm)
Measured in
diopters
In depth
Unlike an extension tube, a diopter adds glass and therefore costs no exposure, but a cheap single-element version introduces chromatic aberration and softens the corners. Achromatic two-element designs correct this and perform well enough for serious work. Power is stated in diopters, and the reciprocal gives the focus distance when the lens is set to infinity.
Examples in practice
A +2 diopter on a 100 mm lens set to infinity focuses at 500 mm, filling the frame with a flower head.