Conjugate focal plane

This comes from the principle of reversibility which states light rays will travel along the originating path if the light's direction is reversed.

[4][5][6] Interchanging the object and image positions does not change the result of the formula.

In a telescope, the subject focal plane is at infinity and the conjugate image plane, at which the image sensor is placed, is said to be an infinite conjugate.

In microscopy and macro photography, the subject is close to the lens, so the plane at which the image sensor is placed is said to be a finite conjugate.

Within a system with relay lenses or eyepieces, there may be planes that are conjugate to the aperture.