How many times have you read an exchange like this online?

Rookie: Hey, I just bought camera XYZ.  What lens will give me the same focal length as a 50mm on a full frame camera?
Old Salt: A 50mm lens is a 50mm lens no matter what camera you put it on.
Good Samaritan: I think you meant to ask about angle of view.  You see, camera XYZ has a crop factor of 1.6, so to get the same angle of view as a 50mm lens on a full frame camera, you would need about a 30mm lens.
Rookie: So a 30mm on my camera is the same focal length as a 50mm on a full frame camera, right?
Old Salt: NO… A 50 is a 50 is a 50.  Stupid kid.

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