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So, you can use both shutter speed and aperture to change the amount of light that strikes the sensor. But why would you choose one over the other? As you have seen, in addition to controlling light, when you change shutter speed, you also change how much moving objects in a scene are frozen or blurred. When you change aperture, in addition to controlling brightness, you also get a change in depth of field. Depth of field is a measure of how much of your scene is in focus. Now, you might think, don't I want all of my scene to be in focus? Not necessarily.
By shortening depth of field, you can blur out the background behind your image. Or by deepening your depth of field, you can ensure that everything in your image is in focus. That's something even your eye can't do. One of your most important goals as a photographer is to ensure that the viewer knows how to read your image. And at the simplest level, that means you want to be certain that they know what the subject of your image is. This is the goal of composition: you arrange the elements of your scene, so that there is a definite subject and background. Depth of field is another tool that you have to guide the viewer's eye.
By softening the background, your subject becomes more prominent. In a landscape shot, the landscape itself is often the subject, so you want to be sure that all of it is in focus. Aperture is the key to controlling depth of field. So now, at last, you should be getting an idea of why there are two mechanisms for controlling light. While both aperture and shutter speed can alter the amount of light that strikes your image sensor, they do this in very different ways, which have very different impacts on your final image. Changing shutter speed alters the sense of motion in an image, while changing aperture alters the depth of field.
Together these two parameters give you a tremendous amount of creative power. But to make use of aperture, you need to know how to manually control it on your camera.
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