I just got the new 24-70 to replace my Mark I version of the lens. Here is my question, given that the new lens has a diameter of 5mm more, does that mean that more light enters the lens?
Any differences there may be will result in negligible changes to exposure. Indeed, if you look at the actual specifications of pretty much any lens, you'll see that the numbers on the barrel are all rounded. A 200 mm lens might actually be a 193 mm lens; an f/4 lens might actually be an f/3.87 lens; and so on.
In addition, glass (and fluorite and the other materials used to make lenses) doesn't transmit 100% of the light coming through. Each lens element absorbs some of the light. A lens might have a focal length : aperture ratio of 1 : 1.4, but the amount of light that makes it through to the camera might only be as much as an impossible theoretically-perfect f/1.8 lens.
Since most people shooting SLRs use the camera's through-the-lens metering and because modern DSLRs have so much dynamic range and therefore exposure flexibility in post-production, those minor variations are insignificant in the real photographic stills world.
However, they
are of concern in the motion picture world, where you've got multiple cameras shooting the same scene from multiple angles and whose resulting images will be composited together in such a way that even minor variations in exposure will be apparent and distracting. And that's why cinematography lenses are calibrated in T-stops instead of F-stops. The T-stop is a measured number of the actual light that makes it through the lens. You might have a lens that, as in my previous example, has an F-stop of 1.4 but a T-stop of 1.8.
It would be interesting to compare the T-stops of the two versions of the 24-70, but really only useful if you're a cinematographer. And I'd be surprised if the two of them differed by more than a third of a stop.
Cheers,
b&
