Focus Shift
A movement of the plane of sharpest focus as a lens is stopped down, caused by residual spherical aberration.
Measured in
mm
In depth
Because the lens focuses wide open and then stops down as the shutter fires, a lens with focus shift is focused under different conditions from those in which it records. The plane usually moves backward, so the error appears as back focus that worsens between about f/1.4 and f/4 and then disappears as depth of field grows.
Examples in practice
A fast 50 mm nails focus at f/1.4 and again at f/8, but places it a few centimetres behind the subject at f/2.8.