Field curvature can make a sharp lens look soft when the wrong assumptions are made. Understanding how sharpness varies throughout the frame will help you get the most out of some excellent lenses that might not test well on flat (planar) test targets.
In previous articles, I’ve explored how diffraction and focus shift can lead to images with degraded contrast and resolution. As if that wasn’t enough to think about, another confounding factor is present: field curvature. In the ideal world, a flat (planar) surface would be imaged onto a flat sensor, and a crisp image would result (a “flat field” lens).
The real world doesn’t work that way for many lenses. In the real world, a desired plane of focus is imaged as a varying curve that can focus sharply in front of or behind the sensor, or both, depending on where we look in the frame. Field curvature can also reverse, swinging alternately in front of or behind the nominal plane of focus, depending on distance from the optical center.
Field curvature is an inconvenient problem, because it calls into question the applicability of lab tests and MTF charts that measure lens performance using a flat (planar) test target. Measurements are typically presented as hard facts describing imaging performance. And so they are—for photographing flat targets at the test distance. But unless your work involves photographing perfectly flat subjects at the same distance, the sharpness of a perfectly flat surface has only a rough correlation with real-world results with three-dimensional subject matter. Some outstanding lenses test poorly due to field curvature; others test extremely well but perform poorly for infinity focus.