DIOPTER CAT

A diopter attached to the front of a lens shifts its whole focus range closer to camera. Rack the lens to infinity and the closest you can hold focus is exactly 1 / D meters from the diopter — that's your far limit. Rack the lens to its own marked close focus (CF) and you get the near limit: CF / (1 + D·CF).

Distances are measured from the front element of the diopter, not the sensor, and this holds regardless of the taking lens's focal length — which is why one chart works for a 25mm and a 100mm alike, as long as you know each lens's close focus. Stack two diopters and their powers simply add: a +1 and a +2 behave like a +3.

It's an approximation — thin, single-element diopter, in contact with the lens, no funny business from the taking lens's own internal floating elements. Good enough to build a pull chart from. Confirm critical focus by eye before you roll.

How you mount the diopter matters. The math above assumes the diopter sits right at the front element of the lens — effectively zero air gap. A clip-on or step-up-ring diopter screwed straight onto the lens is a close match to that assumption. A diopter dropped into a matte box is a different story: it's sitting some real distance forward of the glass, and the farther forward stage you use, the bigger that gap gets. The bigger the gap, the more the true near and far limits will drift from this chart, and the effect is worst exactly where you can least afford it — strong diopters at close focus. Standard practice, and the safest one: mount your diopter in the innermost matte box stage, as close to the lens as the box allows, and treat anything shot through an outer stage as an estimate to be confirmed by eye.

Anamorphic glass. A standard round (spherical) clip-on diopter adds the same power in both meridians no matter what the taking lens does behind it, so the near/far numbers here are a reasonable planning start even on anamorphics — the squeeze happens further back in the lens, not at the diopter. Two things to watch for: purpose-built anamorphic or cylindrical diopters and expanders exist precisely because a spherical diopter isn't the whole story at close focus on some designs — those add power asymmetrically between axes and don't follow this chart's math at all, so lean on the manufacturer's own numbers for those. And separately, anamorphics generally show more breathing and softening through a diopter than spherical glass does, so double-check image quality, not just focus, before committing to a strong diopter on anamorphic primes.