Focal Length
The distance from a lens's optical centre to the sensor when the lens is focused at infinity, which determines magnification and angle of view.
Formula / rule
1/f = 1/u + 1/v
Measured in
mm
In depth
Focal length is a property of the glass alone, so a 50 mm lens is 50 mm on every camera ever built — what changes is how much of its image circle the sensor crops. Doubling the focal length halves the angle of view and doubles the size of a distant subject in frame. It is engraved on the barrel because almost every other lens property follows from it.
Examples in practice
A 24 mm lens takes in a whole room; a 400 mm lens fills the frame with a bird twenty metres away.