If you read photography press or internet forums, eventually you will come across talk of lens focal lengths that are regarded as “portrait lenses.” As surely as there are lenses that can provide beautiful portraits, there are caveats to that notion, the most important of which is that most lenses can produce excellent portraits of people when used by talented photographers.
The most obvious concept common to portrait lenses is that they be flattering to the subject. The person being photographed shouldn’t appear foolish or ugly, distorted or deformed (except when the editorial content calls for it). Part of achieving this “flattering” look is being able to shoot the human form from a distance that is comfortable, from a distance that is approximately the same as you would view someone in a normal conversation. We see and interact with other people at this distance all the time, and our brains understand this distance best when dealing with the human face. This distance is also not uncoincidentally a comfortable distance from which to work with your subject.
To that end, we need a lens that will allow us to fill up the frame with the desired amount of human form from this normal conversational distance, which most of the time is roughly five to fifteen feet. From that distance if you want to fill the frame with a human face, you need something in the ball park of a 70mm to 135mm lens. To fill the frame with head-to-waist, something in the 50-75mm range might work, and head-to-toe might call for a 35-50mm lens or so. There are a lot of variables, and each portrait will be unique (unless you go to Wal Mart, where each portrait is pretty much the same).
Another factor to consider when photographing people is the background, and how it is rendered by lens, aperture, and working distance. Much of the time we like the background to gracefully disappear into a form that sets tone and gives us a small amount of contextual information, but does not take the viewer’s eye away from the subject. We try to do this often by using large apertures, longer focal lengths, and closer focusing distance, all of which result in shallower depth of field, making the background softer and less intrusive.
Another less tangible aspect of achieving flattering, unobtrusive backgrounds is the concept of “bokeh,” which is a Japanese word that means “blur” or “haze,” and in the photographic community is used to describe the character of the out-of-focus portion of images. Some lenses are said to have “good bokeh,” while others have “bad bokeh.” I personally use the term “ratty bokeh” to describe lenses that have distracting out-of-focus areas. Certain lenses are noted for having really beautiful bokeh, like the expensive 85mm offerings from Nikon and Canon. In fact, the manual focus version of Nikon’s 85mm f/1.4 is regarded as the gold standard by which bokeh is judged.
In the end, of course, how a lens renders the out-of-focus areas of a photograph is up to the photographer to judge, and one man’s bad bokeh is another man’s perfect background.
For what it’s worth, most of my favorite portraits in my portfolio are not planned, studio-style shots, but occasions when light and lens and subject have come together and I was ready to see it and photograph it.