>>3117060Have to remember that no glass is perfect, and so some of the light entering the lens will bounce around the lens at odd angles and just be converted to heat. How much light actually makes it to the lens is called a T-Stop. If you have a lens with 3.5 F-Stop, then in a perfect world it would have 3.5 T-Stop, but we don't live in a perfect world. So it might really be 4.5 T-Stop in reality, on a lens labeled 3.5 F-Stop. How old lenses compare to new lenses is something I don't know but I do know that modern IS (like in Canon EF lenses) adds additional lenses for the light to pass through and causing more attenuation of light, however minor this attenuation might be.
