Depth of Focus
The tolerance in sensor position, on the camera side of the lens, within which the image stays acceptably sharp.
Formula / rule
≈ 2Nc
Measured in
µm
In depth
Depth of focus is the mirror image of depth of field, measured in micrometres rather than metres, and it explains why sensor alignment and lens mount flatness matter so much. At f/1.2 on a high-resolution sensor the tolerance is only a few dozen micrometres, which is why a mount tilted by a fraction of a degree produces one soft corner.
Examples in practice
At f/2 with a 30 µm circle of confusion, the sensor must sit within about 120 µm of the correct plane.