What is the focal plane?

The focal plane, or principal plane of focus, is the plane upon which the lens is actually focused. This distance is often represented on the lens barrel upon the distance scale. In a perfect lens, all of the light rays emanating from any point (meaning a "singularity", not a "place") that lies in that plane of space will converge in exactly the same place upon the film. ( . ) Any point before or beyond that plane will reproduce as a circle. ( o ) If the circle (known as a circle of confusion) is small enough, it is acceptably sharp to our eye, so it also appears to be in focus. This is what gives us 'depth of field', which is the distance before and after the focal plane that appears to be sharp. Of course, this perception depends mainly on the degree of magnification of the image. If these out-of-focus circles are enlarged enough, we can detect they are not actually points.

The focal plane is not to be confused with the film plane.

First answer by Micron. Last edit by Micron. Contributor trust: 64 [recommend contributor]. Question popularity: 1 [recommend question]