Which aberration is described by field curvature causing focal shift across the image plane?

• A. Curvature of Field
• B. Distortion
• C. Coma
• D. Spherical aberration

Answer: (A)

Field curvature is when the best focus sits on a curved surface rather than a flat plane. Because the focal plane isn’t flat, different parts of the image (center vs. edge) come into focus at different distances from the lens. With a flat detector, this means sharpness varies across the field—you get a focal shift from center to edge as you move across the image. This is distinct from distortion (magnification error across the field), coma (off-axis points get comet-like tails), or spherical aberration (axial focus varies with ray height but tends to blur across the whole image rather than along a curved focal surface).