[poider (Peter)]

Originally Posted by JA

The focal length of an optic is the distance it takes the optic to cause a parallel bundle of light rays entering the optic to all come to pass through one point: the focal point. Different optical designs: reflectors or refractors or refractors with more glass or glass with higher indicies of refraction will bend the light more and hence focus in a shorter distance and therefore not need to be physically as long.

Best
JA

Thank you JA.