Field Curvature

What is field curvature?

Field curvature is the focusing of an image on a curved surface, rather than on the idealized image surface (a plane). This means that points on a plane surface are imaged onto a curved surface.

Why does field curvature exist?

A planar object perpendicular to the axis will be imaged as a plane only in the paraxial region. This results in a curved image surface, called a Petzval surface, that is the focus of the object. For a positive lens, the surface curves inward (figure 1). For a negative lens, the surface curves outward (figure 2).

How do you get rid of it?

For there to be zero resulting field curvature, the system must meet the Petzval condition:

This can be achieved with a combination of positive and negative lenses.

Hecht, Eugene, Optics, 4th edition, San Francisco: Addison-Wesley, 2002.