Standard Lens
A lens whose focal length roughly equals the diagonal of the format, rendering perspective close to unaided human vision.
Formula / rule
f ≈ frame diagonal
Measured in
mm
In depth
For full frame the diagonal is 43 mm, which is why 50 mm became the default kit lens for decades — near enough, and easier to build fast and cheap. A standard lens neither expands nor compresses space, so it leaves the photographer nowhere to hide behind an effect, which is precisely why it is such a good teacher.
Examples in practice
The 50 mm f/1.8 on 35 mm film, the 80 mm on 6×6 and the 25 mm on Micro Four Thirds all give the same natural perspective.