The slice

format

focal length (mm)

aperture

focused at (metres)

—

The circle of "sharp enough"

Strictly, a lens focuses at exactly one distance; everything else is blurred. The reason photographs don't look that way is that small blur is invisible: a point can smear into a tiny disc before an eye at arm's length from a normal print calls it soft. The diameter where the calling happens is the circle of confusion, and it is the only opinion in this calculator — everything downstream is geometry.

The convention scales with the negative, because big negatives are enlarged less. This page uses 0.030mm for 35mm film and full-frame digital, 0.019mm for APS-C, down to 0.1mm for 4×5 sheet film — the figures the classic depth-of-field scales were engraved from. Print huge or pixel-peep and your personal circle is smaller than the convention's; the slice shrinks accordingly. The maths is exact; "sharp" is a social agreement.

Zone focusing, the meterless autofocus

The old street-shooting trick, and the hyperfocal number is its engine. Set the aperture first — say f/11 — then turn the focus ring to the hyperfocal distance the calculator prints. From half that distance to infinity is now acceptably sharp, and the camera has become a box you merely point: no focusing, no hunting, no missed moment. A 35mm lens at f/11 on 35mm film zone-focuses from under two metres to the horizon.

The same thinking works close in. Portrait at three metres with an 85? The calculator will show you centimetres of depth at f/2 — which is why film portraitists focused on the near eye and said a prayer — and half a metre at f/8, which is why group photos happen at f/8.

Fine print on focus

depth splits unevenly — roughly one-third in front of the subject, two-thirds behind at portrait distances, evening out to half-and-half up close

diffraction pushes back — beyond about f/16 on 35mm (f/8 on small sensors) the whole frame softens even as the slice deepens; the engraved scales knew nothing of this

focus-and-recompose bends the numbers — swinging the camera after focusing moves the plane; at wide apertures and close range that alone can spend your whole slice

infinity means the convention's infinity — when the far limit reads ∞, distant subjects are inside the agreed blur, not optically perfect

exposure still owed — stopping down to f/16 for depth costs four stops from f/4; the meter and its calculator settle that bill

← back to the light meter — depth decides what's sharp; light decides everything else.