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In program mode, when you half-press the shutter button on your camera, your camera meters the scene and calculates a shutter speed and aperture that will be adequately exposed, neither too bright nor too dark. In shutter priority mode, as you've seen, you can dial in a shutter speed, and the camera will choose a corresponding aperture that is neither too bright nor too dark. In other words, in both of these modes, your meter protects you from over- or underexposing. Well, let's look closer at what happens when I change shutter speed, when I'm in shutter priority mode. I'm back here with a scene we saw before, the toy with the spinning spaceships.
Let's say I want to shoot it to freeze motion, so I dial in a shutter speed of say 1/500th of a second. So here I am in shutter priority mode. I'm dialing up to 1/500th of a second. When I half-press the meter, I see that the camera has chosen an aperture of f/2.0. Now let's say I decide I want to blur the motion instead, and so I want a slower shutter speed. So I'm going to slow my shutter speed by one stop. Remember, a stop is a doubling. So if I switch from 1/500th to 1/250th, which is a shift from faster to slower, I've changed my shutter speed by one stop.
1/500th halved is 1/250th. So I'm going to slow my shutter speed down to 1/250th, and now I've gone to f/2.8. I half-press the shutter button again, I see that f/2.8 is what the camera chooses. My light has not changed, but the camera has chosen a new aperture. When I slowed my shutter speed by one stop, I allowed twice as much light to reach the sensor. So the camera picked an aperture that is one stop smaller than what I used before. My shutter speed allowed twice as much light, so to compensate, the camera has picked an aperture that blocks twice as much as my previous aperture.
If we go back to our Aperture chart, we see that an aperture change from f/2.8 to f/2.0 is a change of one stop. Remember, every time I meter, the camera calculates an exposure that will yield a good level of brightness. Since my light has not changed between these two shots, the camera is aiming for the same level of exposure. So when I changed shutter speed, the camera had to choose a different aperture from before, so as to preserve the same level of brightness. It can do this because shutter speed and aperture have a reciprocal relationship.
If I change one parameter in one direction, I can change the other parameter by the same amount on the other direction, and preserve the same overall level of illumination. Now this is not something unique to shutter priority mode. It's simply a fact of exposure. For any given level of illumination, there are many combinations of shutter speed and aperture that yield the same overall brightness. This is great news for you, because it means that one of those combinations will probably help you achieve the image that you see in your head. However, this also means that you cannot over- or underexpose in shutter priority mode.
If you change the shutter speed, the camera will always pick a corresponding aperture that yields a good exposure. If it can't, it will flash it at you, as you saw earlier. Now this doesn't mean that your camera will always take perfect exposures. Your light meter can still be confused. Sometimes, it will come up with an exposure that over- or underexposes things. But if you want to intentionally over- or underexpose beyond what your meter suggests, you'll need to learn some additional controls that we'll cover later.
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