Making Better Images With The Photographic Triangle
Cameras these days generally do a good job of automatically determining exposure, but that doesn't mean you have to accept what yours comes up with. By understanding the photographic triangle, you can take control of exposure.
Back in the film days, photographers mainly talked about exposure in terms of aperture versus shutter speed. The Law of Reciprocity dictates that if you adjust one of these two variables, you must change the other in the opposite direction to compensate. But unlike with film, you can easily change ISO now too which gives us three variables that affect exposure. If you take the concept of reciprocity and stretch it to three points rather than just two, you get the "photographic triangle."
There's a reason why shutter aperture, ISO and shutter speed can all be measured in stops. It's because they are all related. If you change one, you have to change one of the others in the opposite direction to compensate. Open up the aperture by one stop and you'll have to cut the shutter speed in half to retain the same exposure. Either that or you can change the ISO setting to half to get the same effect. Each of these will have other impacts on how your photograph comes out, but just in terms of exposure, both are equal.
So suppose you have your camera set on automatic exposure. It's OK to admit it. Nothing wrong with letting your camera do the hard work of determining exposure for you. But before you press the shutter release, think about what other impacts the choices it made will have.
If your camera determined the exposure should be f/5.6 at 1/4 second you can be fairly confident that this will give you a good exposure. But what else will it affect? An aperture of f/5.6 is fairly wide and will result in less depth of field than a narrower aperture would. Depending what you need to be in focus, this may work fine, but it may not for other scenes. If you feel you would benefit from more depth of field, you might decide to change the aperture to f/8, a change of one stop. But if you do, you'll also need to go with a longer shutter speed of 1/2 second, or else boost the ISO by one stop to compensate.
On the other hand, 1/4 second is long enough that if the wind is blowing you may see some motion blur. If you are photographing a field of wildflowers, you may end up disappointed, but if your subject is a waterfall, even 1/4 second may not be long enough to do it justice. Decide to go with a shutter speed of 1/15 second to keep your wildflowers in place while you photograph them and you'll need to open up the aperture two stops to f/2.8 to keep the same exposure. If your lens only has a maximum aperture of f/4 or you don't want to sacrifice depth of field, you can boost the ISO by two stops to keep exposure the same. On the other hand, if you have your camera pointed at that waterfall and decide to go with 1/2 second exposure to blur the water more, you'll need to stop your aperture down by one stop to compensate, or lower your ISO by that same stop.
Some cameras have a variable program mode or similar that can help you make these sorts of adjustments. You're camera can't know everything though, and even though it may come up with a good exposure, it won't necessarily come up with a good image. That's where you come in. Even the fanciest of cameras can't assess what is important in a scene as well as you can It's up to you to make each image the best it can be.