Focal length

The focal length is the distance between the optical centre of an optical system (lens etc.) and its focal point.

In paraxial optics, the image focal length is the distance between the principle image plane and the image focal point. We also define the object focal length, opposite in sign, and equal to the image focal length as an absolute value if the object and image media have the same refractive index.

In the special case of an instrument for observing objects at infinity (telescopes) and if the objective can be assimilated to a thin lens, the focal length is also equal to the distance between the objective and the intermediate image plane.