I wrote the following yesterday:
"There are two relationships in a photograph that give it a distinct look. The distance from the camera to the subject and the distance from the subject to the background. The second factor determines how much blur the image will have. Different length lens give these two related factors their unique photographic qualities."
I would like to go into a little more detail about how this affects the look of a photograph.
There are many factors that invest a photograph with it's final look. One of the most important factors is the relationship defined above. When the subject to background distance (B) is less than (A) the subject to camera, very little blurring occurs.
In this image of an Osprey in flight, the background is only slightly blurred and is easily identifiable. It does not detract from the photo.
This photograph of a White-throated Sparrow shows surroundings (both foreground and background) that are more distracting because of the ground litter, but you can see that as the distance behind the sparrow becomes greater, so does the blurring.
I took this picture last summer with the idea of illustrating how this principle works in a real situation on the blog. This is a picture of the woods at the edge of my front lawn. It was roughly fifty feet from the camera. The area highlighted in yellow is approximately the area of the background in the next photo.
The hummingbird was only about eight feet from the camera while the distance to the trees remained the same. Because the bird was so much closer, the background becomes a simple wash of color. The blue sky in the upper left corner of the last image corresponds to the blue circle in this photo. The green circle beneath corresponds to the green circle of light in the image above. The dark area on the right is the tree in the other photo.
Knowing how this relationship works allows the photographer to take advantage of the "raw" ingredients that are available to produce the desired look. Under the right circumstances, the background can even be aligned to select a specific color as a backdrop. That was what I did in this image. I changed the angle of the photo to incorporate sunlight streaming through the trees in the background.
Sparkles of light and small holes of light in the background can be incorporated into the image to become something called "bokeh," which is defined as the visual quality of the out-of-focus area. This can greatly enhance the interest of a photograph as I think it does in the image of an Eastern Bluebird.
The distance relationships described here become part of an arsenal of tools to create better pictures as they are incorporated into the photographer's experience. It becomes second nature to look for situations where you can take advantage of this.