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In Program mode, when you meter a scene by half-pressing the shutter button, the camera calculates an appropriate shutter speed, and aperture, and maybe ISO, if you're set to Auto ISO. There will be times, though, when you know that you're going to want a lot of control of aperture. Maybe you're shooting portraits, and you know that you want them to all have shallow depth of field, so you want to make certain that the camera is always using a wide aperture. Or maybe you're shooting landscapes, and you know that you want really deep depth of field in all of your shots, so you want to make certain that you're always using a very small aperture.
Or maybe you're street shooting, and as you're moving around quickly, you're shooting different subject matter, and so you're changing your mind about depth of field, and so you want to be able to really quickly and easily change from a big to a small aperture. In Aperture priority mode, you can choose the aperture that you want, and when the camera meters, it will automatically pick a corresponding shutter speed that will yield a correct exposure. By now you should already know how to change to Aperture priority mode; that's Av on your mode dial, and that's aperture value, so I am just pressing the lock button, and turning the mode dial.
Now watch what happens when I meter, and the camera is metering at f/5 at a 60th of a second. When I turn the main dial, what's happening is I'm changing the aperture; I am in control of aperture, and the camera is automatically calculating a new shutter speed. So notice that as I go to a smaller aperture, my shutter speed is getting longer, because with a smaller aperture, it needs to let in more light. As I go the other direction, my aperture gets bigger, my shutter speed is getting shorter, so there is less light coming in.
So I've got all the control of aperture that I want, and the camera is automatically calculating shutter speed for me. Now, I am going to let the meter time out here, and notice I can still change my aperture. So I can do this even before I meter. If I am going into a scene, and say I know that I want deep depth of field, I can just go ahead and drop it here on maybe f/11, and it will just stay there until I change it, and now my metering will go accordingly. I'm currently set on ISO 100; watch what happens if I change ISO to Auto.
Now I'm metering at f/11, my shutter speed has gone up to 100, because the camera has decided it can go at ISO 800. And as I change here, notice that shutter speed isn't really changing; it's just manipulating the ISO. The idea here is it's trying to keep my shutter speed at something that's going to be good for handheld shooting. So Auto ISO is very smart when I am working in priority modes. Of course, it can go quite a long ways here; I am up to ISO 3200. If you decide that you don't like ISO going that high because of the amount of noise, you can actually reprogram the Auto ISO feature to not go beyond a certain point, and you can see that in a separate movie.
Aperture priority doesn't allow you to take any shots that you couldn't take in Program mode using program shift. Instead, it just provides you with a speedier way to get to the aperture-based exposure settings that you know you need.
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