Back Focus
A calibration error in which a lens consistently focuses slightly beyond the intended subject.
Measured in
mm
In depth
Back focus is the mirror of front focus and has the same cause and cure. It is often noticed first in portraits, where the ears are sharp and the eyes are not — a diagnostic pattern worth recognising, since the instinct is to blame technique. Consistency across many frames is what identifies it as calibration rather than error.
Examples in practice
Every portrait from one particular lens shows the eyelashes soft and the hairline crisp, at every distance.