Quote:
Originally Posted by poider
So how is the focal length determined, I have a 600mm long refractor with a focal length of 600 but my sigma 150-500mm is no where near 500mm it is just over 350mm long
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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