Field Curvature
A defect in which a lens focuses a flat subject onto a curved surface rather than the flat plane of the sensor.
In depth
With a curved field, focusing on the centre leaves the corners front- or back-focused even though the subject is genuinely flat. It explains why a lens can look soft in the corners on a test chart yet perform beautifully on a three-dimensional scene where the corners are further away anyway. Macro and copying lenses are specifically corrected flat.
Examples in practice
A landscape lens with inward field curvature nails a distant horizon at the centre but softens the corners of a flat-copied map.