Which aberration is also known as 'Power Error'?

• A. Longitudinal spherical aberration
• B. Radial
• C. Curvature of Field
• D. Coma

Answer: (C)

Off-axis focus variation happens because the image surface where sharp focus occurs is not flat; it curves. This means points away from the center don’t all come to sharp focus on a flat retina, producing a blurred or defocused edge as you move off-axis. That curved focal surface is called curvature of field. In some texts it’s also referred to as a “Power Error” because the effective optical power changes with field position, causing the focus to shift across the field. So, this aberration is the one that describes the field-dependent change in focus, hence why it’s labeled curvature of field.

Longitudinal spherical aberration affects focus along the optical axis, not the curved layout of the focal plane across the field. Coma produces off-axis blur with comet-like tails but isn’t about the focus surface’s curvature. The term radial isn’t the standard designation for this particular field-related focus issue.