What aberration causes a planar object to project as a curved image?

The correct answer identifies curvature of the field as the phenomenon responsible for a planar object projecting as a curved image. This aberration occurs when the focusing of light rays through a lens does not fall on a flat plane but instead forms a curved focal surface.

In practical terms, when an object that is supposed to be flat is imaged, the light rays coming from different parts of the object may converge to a focus at different distances. This means that parts of the image will be in focus, while others will be out of focus, leading to a curved appearance of the image rather than a uniformly flat one. This can particularly be observed in wide-angle lenses where the image's periphery tends to show more curvature.

Field curvature is closely related but is often used interchangeably with curvature of the field; however, it can refer more specifically to the resultant image on the sensor or film not lying flat. Chromatic aberration, on the other hand, is related to the inability of a lens to bring all wavelengths of light to the same focal plane, resulting in color fringing. Lens distortion involves more geometric misrepresentation of the image, such as barrel or pincushion distortion, but does not specifically create a curved projection of a planar object.