For a complex photographic lens, the 50mm is not usually measured from any obvious physical glass surface. In a simple thin-lens ray diagram, focal length is the distance from the lens to the focal point, but real camera lenses have multiple elements, so there may be no single physical spot you can point to and say “the 50mm starts here.”

In practice, focal length depends on the overall optical design, and for interchangeable camera lenses it is typically specified at focus toward infinity. It can also change somewhat with focus distance, so a lens labeled 50mm may not behave as exactly 50mm when focused close.

So in a ray diagram of a real multi-element lens, you would usually model the whole optical system using its effective principal plane(s), not measure from a particular element. If you tried to check it physically, you could project light through the lens and find where it comes to focus, but that still would not identify a single universal internal “50mm mark.”