After learning how aperture, shutter speed and ISO work individually, you have to learn how they work together. They are what are known as the three pillars of photography, and their workings are interconnected. Each individual setting has an effect on the other two, and while this can be confusing, learning how to master this will result in perfect shots using manual setting. These settings are changed depending on the lighting, the situation that you are photographing and how you want your image to turn out. Learning how to prioritise your settings is what will make your photographs stand out from those who shoot on automatic or priority mode.
Using these three settings together in order to correctly expose your image is called light metering. When looking into the view finder of your camera, you will be able to see a line at the bottom with the numbers -2, -1, 0, 1, 2. This line shows how much light or how little light is coming into your camera, and it is known as the light meter. The light meter shows what the camera thinks is the correct exposure for your image. If your marker is on -1 or -2, it means that there is not enough light coming into the camera. You would then have to change one of your settings to allow for more light. Conversely, if your marker is on 1 or 2 it means that there is too much light and you need to adjust one of your settings to allow for less light in your image. Choosing which setting to change is the difficult part of light metering, but with practice, it soon becomes second nature.
Firstly, you need to decide how you want your image to come out. If you are shooting a subject that is in fast motion and you want to capture crisp movement, you would have to have a high shutter speed of about 1/500th of a second. As you have decided on this setting, you would then have to change the aperture and ISO in order to suit the lighting of your situation. For example, if you are shooting a sporting event in the late afternoon or early evening, you would have to use your settings to let in as much light as possible. This is because your high shutter speed will not allow for a large amount of light to enter into the camera. You will then have to make your aperture as wide as possible so that enough light can enter. However, this will mean that you would have a shallow depth of field. If the depth of field is too shallow and does not get the whole player in focus, you will then have to put up your ISO. This will allow for more light in your image whilst still have an acceptable depth of field and a fast enough shutter speed. Be careful how much you put up your ISO, as it might cause your image to become noisy. Alternatively, if you want your image to have a small depth of field, then you will have to decide on your aperture first, and then you would decide on the rest of your settings according to the lighting of your situation.
Light metering is also useful in situations that have contrasting lights. For example, if a group of people were sitting in the shade of a tree on a bright day full of sunshine, then you would have to meter wisely. Adjusting the light to the people in the shade will mean that the background will be over exposed. Adjusting your settings to the background will mean that the shaded area will be too dark and will have no detail. In this situation, you would have to decide what is most important to you in the image: the group of people or the background. It is often wise to choose settings that are in between; not too dark in the shade and not too light in the background. However, if this is not possible, you should meter for what is most important in the image. If you have any sort of editing program, you can light meter for one and fix the other later on in post production. In this case it is better to expose correctly for the brightest part of the image, as shadow is always easier to bring back in post-production, and something is over exposed it cannot be fixed. Just remember that over editing in terms of fixing light issues is dangerous, as often your image will become noisy if you have attempted to bring back information on your photograph that is not there.
Light metering is therefore about simply choosing what is most important to you in your image, like capturing crisp movement or having a small or large depth of field. Light metering is easy once you have a good knowledge of aperture, shutter speed and ISO. Once you are familiar with all three, as well as having a good knowledge of your camera, it will become second nature.
The link below shows how the light meter on your camera works. A high shutter speed does not let in enough light for the settings on the camera. The light meter therefore shows -2 when the fastest shutter speed is chosen. The slower shutter speed lets in too much light for the settings on the camera, and the meter therefore shows +2 on the slowest shutter speed. Ideally, you want the meter to be on 0, as this means there is the correct amount of light for your settings. Follow the link for a better understanding of how the light meter works.