"Since the background is as important as the subject, you mustn't let it default by chance. You must control not only vertical and horizontal, you must be aware of the depth of field (or lack of it) that you want in the background."

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As you see something that yo want to shot and it's bearing down on you, it's important to start framing long before the subject gets close to you. The light will reveal itself possibly long before you want to take the image, but you have to wait until the picture comes to you, and if you've been anticipating carefully when the subject will be in position, the background will have been figured out in advance.

— Jay Maisel