Focal length is another one of those science-y photography concepts that’s a little hard to explain so I’d like to give a big thanks to Digital Photography Review for helping me explain it. Focal length is the distance from the optical center of the lens to the focal point which is projected on the sensor if the subject is in focus.
Yeah, doesn’t make much sense to me either.
The long and short of focal length (pun kind of intended because I love bad puns) is that the focal length of a lens determines what types of photographs you take with them.
If a lens is 35mm or less in maximum focal length, it’s a wide angle lens. 50mm is considered a normal lens, and if the focal length at its maximum is 85mm or greater, it’s a telephoto. My 150-500mm Sigma is considered a supertelephoto.