I think that most photographers choose the focal length they shoot at primarily based on the desired size of the subject in the frame. Want to get a head shot from far away, use a long lens. Want a full body shot in a confined space, better have a wide-angle lens handy. Today, I'd like to talk about another criteria to consider in choosing your focal length: subject isolation.
In portraiture we usually want the subject to stand out against the background. Experienced portrait photographers don't just look at the subject, they look at the background as well. They try to compose their images so that their backgrounds aren't that too cluttered or colorful or so interesting that they draw the viewer's attention away from the subject. They also tend to shoot at wider apertures to minimize depth-of-field, which allows them to blur the background while keeping the subject in sharp focus. Longer focal length lenses can help on both counts. To explain why, we'll need to introduce a bit of geometry.
