Focal length Calculatortelescope

Orthoscopic eyepieces have four lenses, of which two are biconvex curved and one biconcave. They offer a high degree of sharpness in the centre as well as at the edge. That's why they are interesting for observing planets and double stars. The advantage: they offer a flat field of view.

This effective focal length calculator helps you select a lens to use with your camera. The calculator takes your target field of view (angle of view) and camera specification then provides the required lens EFL.The calculator uses the camera lens focal length equation. This formula is different than the lens maker's equation which is intended for simple thin lens elements. The Optical Effective Focal Length (EFL) should not be confused with the Mechanical Back-Focal Length (BFL). The EFL determines the field of view of the camera while the BFL determines whether a lens will focus on a camera.