I had always thought that there was supposed to be little to no perceptable difference between different lenses when it came to exposure. In other words, f/8 at 1/250 second with one lens should give the same exposure as f/8 at 1/250 second on another lens. But, for whatever reason, it seems like there is quite a bit of variance between my lenses. For instance, my 50/1.4 seems to overexpose compared to my 24-70/2.8, probably by around half a stop (or maybe even more). Is this sort of thing normal?