Discussion about composition goes into all sorts of things, from the "rule" of thirds to watching out for distractions to leading lines and so forth. All good. Anything that helps you see the scene better photographically will make you create better compositions.
A big challenge we all face with composition is that we see the world differently than the camera does. We see subjects, the camera could care less about subjects. So we have to train our eye to see what the camera sees, not just what we see, and this is what a lot of composition instruction seeks to do.
There is one place where how we see subjects and the camera sees subjects that is rarely included in composition discussions, yet it is very important. That is emphasis. We do something unique with our eyes and brains – we will look at a scene and emphasize what is important so that we can make sense of a scene. For example, if you look at a crowd, you do not see the crowd simply as a mass of people. Your brain will use the visual from your eyes in such a way as to emphasize certain aspects of the scene. This helps you pick out a friend in a crowd and is also why some oddball in a crowd stands out.
The camera does not naturally do that. The camera puts equal emphasis on everything that shows up on the sensor. So a photo of that crowd will be just that, the crowd, and a friend or some oddball will not automatically show up in that image. A challenge I see in many photographer's images is that they concentrate so much on the subject that they don't see the actual photograph being taken by the camera.
Composition is ultimately about communication – it helps the viewer understand what you think is important about a scene or a subject. So we have to help the camera emphasize what is important in our images so that they communicate what we expect them to communicate. That emphasis will structure the photo so that it can be understood better.
There are many ways to emphasize elements of a composition, such as your main subject in a larger scene, but I am just going to talk about three very important aspects of emphasis that strongly affect an image and its composition. They are depth of field, size and light.
Depth of field is an easy way to create emphasis simply by choosing shallow depth of field. Any time you can contrast a sharp subject against an out-of-focus background, you gain emphasis of that subject. You can have two images that are identical in every traditional aspect of composition, yet the compositions will be dramatically different simply because of the change in depth of field. Deep depth of field emphasizes the whole scene and can distract from your subject. Shallow depth of field reverses that, de-emphasizing the scene and putting attention on your subject. It actually mimics the way our brains work with our eyes to emphasize things in a scene that otherwise would all blend together.
Too often photographers worry too much about getting depth of field, especially for close shots, and not enough about emphasis. Sure, the subject might be sharp, but then the rest of the photo often fights with the subject because there are too many details competing for the viewer's attention.
Size is something that was a common way of emphasis before zoom lenses. Today, people have gotten so used to standing in one place and zooming that the use of size for emphasis is less well known. One aspect of size, such as a contrast in size of a small object next to a big object or a small object next to a large space, is not affected by zooming or not zooming. However, changing size emphasis from near to far is and that can be a very helpful control for emphasis. You cannot do that simply by zooming.
Size contrast for emphasis can be affected by how you use focal length. If you get close to your subject with a wide-angle focal length, the subject will be large and emphasized compared to a smaller background. That can be a very strong form of emphasis and one that I like a lot in nature photography. A wide-angle up close will make the subject large and shrink the background (top photo of thistle flower strongly shows this), adding emphasis and also environment.
A telephoto also can be used, but in a different way. Telephotos will enlarge the background compared to the subject. Sometimes a background can have too much "stuff" in it, but you might find an area of darkness or a solid color. By backing up and using a telephoto, you can enlarge that darkness or color so that it fills the background behind your subject in your image. That creates emphasis now because the large size of the background simplifies it behind your subject.
Light is often a big help in emphasis. Sometimes we will shoot at certain times of day or from only certain angles to the light in order to get the right emphasis. Any time we can use light to create contrast between subject and background or surroundings, we gain emphasis. This could be as simple as the subject in the light and the background in shadow or the subject in shadow against a light background.
The corollary is that light can also deemphasize a subject when the light and shadow are in the wrong places. That can simply be light and shadow creating a pattern over the subject that obscures it to the camera even though we can see the subject just fine. Or it can be light that creates distractions in the background that attract the viewer's eye away from the subject. Bright, and especially contrasty, areas in an image will always attract the viewer's eye, so we have to be careful how we use them, or remove them, in our photos.
So next time you are thinking about composition, remember emphasis. Many of the ideas of composition don't include that, yet it is key to working photographically.