Visual Plane
every picture states the point it is correct from
Concept

Crop factor — where it appears

The ratio of the 35 mm frame's diagonal to a smaller sensor's, used to convert focal lengths and exact only on the diagonal.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

full frame · 39.6°APS-C · 26.6°Micro Four Thirds · 19.6°1 inch · 15.0°phone (1/1.7″) · 8.7°one 50 mm lens · the angle is a property of the rectangle behind itMicro Four Thirds: 2.00× diagonal, 2.08× wide, 1.85× tall50 mm across five formats39.6° down to 8.7°

A focal length is not an angle

Fifty millimetres means nothing until a rectangle is named behind it. The same lens is 39.6° across full frame, 26.6° across APS-C and 8.7° across a phone sensor — and the distance the resulting print is correct from depends on the ratio of the two, so two cameras matched on angle agree exactly whatever their formats.

sensor · format 3 m, 50 mm1.50 m, 25 mmsubject ×1.000000 · background ×0.526 · zoom alone would give ×1 for bothnear-to-far ratio 10.00 → 19.00changing the focal length leaves it at 1.000000000000

Stepping closer is not zooming

Changing the focal length leaves the ratio between any two things in a picture exactly alone — to twelve decimal places, at every focal length there is. Moving changes it. Hold the subject's drawn size across a step from 3 m to 1.5 m and the background halves, which is the whole of the shot everybody knows and nobody derives.

sensor · dollyzoom

Named alongside it

The objects these essays reach for when they reach for this one.

Demonstrationfield of viewFocal lengthStation pointViewing distanceAspect ratioDepth cueDiminutionForeshorteningPicture planeSensor formatSubtended angle

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