For the heck of it, I tested my EF-S 17-85mm f/4.0-5.6 IS USM kit lens on a 400D with a borrowed EF 24-105mm f/4.0L IS USM lens recently.
I mounted both lenses in turn on my camera, mounted on a tripod, and aimed it at a controlled indoor scene with both lenses set to 50mm focal length and stopped down to f/5.6. Then I metered the scenes, obtaining a shutter speed of 1/8 for the 24-105 and 1/10 for the 17-85.
What the? If anything I would have expected the much larger and more expensive L lens to be more light efficient and meter a faster shutter speed! I don't know how to explain this - sure, the 24-105 is older than my kit lens... but when we compare lenses we usually say the f/2 is brighter or faster than the f/4 etc... That's true in one sense, i.e. the f/2 can theoretically capture 4 times as much light as the f/4, but ONLY if you open it up. Are we missing some quantity associated with efficiency of light transmission through the entire lens?
To me, it seems that for some reason the L lens is less light efficient than the non-L lens. Under low-light of course it would win because it has the extra stop or so to play with if I used it wide open, but what if I was hand-holding, in relatively low-light, and I cared about maximising my DOF and didn't want to use it wide-open? Wouldn't that mean that the non-L lens has an advantage?
Correct me if I'm wrong, but f-number is simply based on the ratio of the physical dimensions of aperture width to focal length, right? Is there another quantity which gives us a measure of the light efficiency of a lens? It seems to me this could be useful as a basis for comparison across lenses.
Any thoughts? I'm not too experienced as a photographer, and this issue is really bugging me, especially since it seems the L lens really ought to have nailed the non-L lens, and not vice versa, in this case.