Field curvature is the aberration that makes a planar object look curved in the image.
Previously, we looked at spherical aberration, which showed that spherical lenses do not focus rays to a unique point. An effect related to this fuzzy focus is field curvature.
The straight object PQ forms a curved image P'Q'. This is because the outer ray (green) has a closer focus than the inner ray (orange), causing the rays through the centre of the lens to intersect the rays through the foci as shown. Constructing intermediate points between P and Q, and their images, subsequently shows curvature.
When field curvature is present, close objects seem to have an inward curve, and far away objects seem to have an outward curve.