Focus
The condition in which light from a point on the subject converges to a point on the sensor, rendering that part of the scene sharp.
Formula / rule
1/f = 1/u + 1/v
Measured in
m
In depth
Strictly, only one distance is ever truly in focus — a single plane parallel to the sensor. Everything nearer or further forms a small disc rather than a point, and the picture appears sharp only where that disc is too small for the eye to notice. Focusing a lens means moving elements to shift where that one sharp plane falls.
Examples in practice
Focusing on a subject's near eye at f/1.4 means the far eye, four centimetres behind, is already measurably soft.