@Brian - I believe Edwin is referring to my comment early in the thread, about how ultrafast lenses (f/1.4, f/1.2) don't provide the light-gathering advantage on a dSLR that most people think. Film was a different story, but digital sensors aren't able to efficiently capture photons at high angles of incidence. So, with an f/1.2 or f/1.4 lens on a dSLR, the camera acutually clandestinely increases the ISO by up to half a stop or more (greater effect with wider apertures and smaller pixels). For example, say you're shooting in Av mode with a 50/1.4 at 1/100 s, f/2, ISO 1600. You change the aperture to f/1.4 without changing the ISO, and you expect the shutter speed to go to 1/200 s - and it does. But, the sensor can't actually capture the full additional stop of light at f/1.4, so instead it adjusts the ISO to 2000. You don't know that from the settings - the EXIF shows the ISO 1600 you set. However, you have an extra 1/3 stop of noise in your shot.