focal plane

Focal plane.

Parallel rays of light that are not parallel to the optical axis will meet on the focal plane. In this example, a simple concave lens is shown.

The focal plane is the flat plane at right angles to the optical axis onto which a lens will focus an image.


Curvature of field results in an extended image forming on a curved surface known as the focal surface.


A focal plane scale is the relationship between angles on the sky, in arcseconds, and millimeters of size at the focus of the telescope, i.e., the number of arcseconds per millimeter.