how many millimetres each size really is, and how much of the frame goes under the knife
Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.
Three sizes in one archive — from a sheet on the fridge to a poster on the wall.
Sheet sizes are ISO 1008; an inch is 25.4 mm by definition, and the millimetres are that inch size rounded. The loss is counted for a centred crop.
The size every laboratory calls «10 by 15 centimetres» is 102 × 152 millimetres in ISO 1008. There is nowhere to put the missing two: the size is not metric at all, it is imperial — 4 × 6 inches, converted and rounded. The whole R series works this way: 13 × 18 is 5 × 7 inches, 20 × 25 is 8 × 10, and not one of them was ever conceived in centimetres.
The consequence is not cosmetic. The 135 film frame, and every sensor that inherited it, is exactly 3:2 — 36 by 24 millimetres. But 102 by 152 gives 1.490, so even the size that is supposed to fit trims a little. The rest trim visibly: 13 × 18 takes 6.6 % of the frame, 20 × 25 takes 16.6 %, and it goes from the sides — that is, together with whatever you deliberately put there.
The question at the counter is therefore not «what size» but «where will the proportion come from». A 4:3 frame out of a telephone behaves the other way round: at «10 × 15» it loses along the short edges, and at 20 × 25 it loses almost nothing. The same paper cuts two cameras in different places.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 6 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.
convert in your head · without a calculator
the ten largest by surface area — as squares at true scale, each inside the next
the A series · from A0 to A6
seven sizes at one scale, with the crop factor computed from the diagonal