Lens Focal Length - Difference between actual and advertised?

[RodneyCyr]

Does anyone have information or web links that discuss the actual (versus advertised) focal length for popular zooms and primes? A quick search of this forum found no other discussions about this issue.

For example, a test report on the Sigma 12-24EX, published by Popular Photograpy shortly after the lens came out, stated that the actual focal lenghs were 12.5mm and about 19mm.

I seem to remember reading many years ago that photography-standards institute in Japan specified that lenses could be up to 5% off their advertised focal lengths. A 20mm lens could actually be 21mm and a 300mm lens could actually be 285mm.

[cmM]

Hmmm.... haven't heard about this one yet.

19mm? That;s like 20% off. If that's true then I expect some protest marches to Canon real soon :D

[DavidEB]

easy subject for confusion. I know that if I tried to measure a lens's focal length I'd probably get the wrong number.

In a multi-element lens, the focal length can change as the distance to the subject changes. Particularly true with lenses that focus by moving the rear elements only. The advertised focal length most likely represents the effective focal length at infinity focus. If you measure focal length the easiest way, by focusing on an object of known size and distance, you won't get the effective focal length at infinity focus, but rather something else.

Another place for confusion is the nodal point. In measuring focal length, you have to find the nodal point, which can move as the distance to subject changes, and also as the lens is zoomed. There's also something called an inner and outer nodal point, and I certainly don't understand those.

So it's real easy to mess up when trying to measure a lens's focal length. I don't even worry about it.

[genewch]

There is always a difference between nominal and real values. To me it's not a big deal. Just try lenses on your camera and see if they are what you expect.