Equivalent focal length is not equivalent.

Matching a frame across two sensor sizes needs a different lens, and everybody knows that much. What the word equivalent hides is that the different lens has a different depth of field at the same stop. A 50 mm T2.8 on full frame frames like a 35.9 mm on Super 35 — and to match the depth as well it has to be T2, not T2.8. Read the other way round, which is how it is usually met: a T2.8 lens on Super 35 is only a T3.9 in full-frame terms.

The engine is free and needs no account. Write the format, the lens, the stop and the distance, and every depth comes back as a distance an actor can move — a total in millimetres, not a pair of limits in metres, because 48 mm is less than a person shifting their weight and “2.48 m to 2.52 m” is the same fact in a form nobody can act on. It computes both halves of an equivalence and says when the matching stop does not exist on any lens. And it makes explicit the thing every other calculator hides: the circle of confusion is a decision about how the picture will be watched, not a property of the camera. A cinema screen is twice as strict as the default rule, so a shot the calculator calls a comfortable 130 mm has 65 mm on the screen it was made for. Every lane ships a worked example that costs nothing. Writing the report needs credits.