Just been reading Understanding Exposure, and there's something I don't get. In the section on low light/night photography, Bryan explains on most images that he meters from the dusky sky - fine. However, he meters at an aperture of, say, f/2.8 to get the corresponding correct shutter speed. He then recomposes the shot, but adjusts the aperture (to f/22 for example) and adjusts the shutter speed by a corresponding amount. For example, if he meters off the sky at f/2.8, but then composes the shot proper at f/22, he adjusts the shutter speed given at f/2.8 by 7 stops/positions to give the correct shutter speed at f/22 (since f/22 is 7 stops from f/2.8). My question is: why not just meter off the sky at the aperture you want to use int he shot, e.g f/22 ? Would that not give the same result ?