Given two lenses with different max apertures but the same size image circle (say, full frame on a Nikon FX DLSR camera) and taking shots at the exact shutter speed and focal length under the same lighting, taken on the same camera, say, f/1.8 and the other f/4.5, if I shoot the faster lens (the f/1.8) at an aperture of f/4.5 doesn't it let in the exact same amount of light as the "slow" f/4.5 lens?
In other words, do I have to have a narrower depth of field (and thus more demand on accurate autofocus) to get the benefit of the fast lens, or if I shoot with a smaller aperture and have more depth of field do I "lose" the speed benefit of the lens (i.e. letting in more light)?
Answer
In theory, both lenses will allow the same amount of light to pass through at f/4.5, so yes, when stopping down, you lose the advantage of fast lens. The actual amount of transmitted light depends on the lens design and is measured in T-stops.
But the fast lens will still make a difference when you look through the viewfinder and your autofocus will also gain from it as the lens is only stopped down to the aperture you set for the shutter actuation time.
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