Obviously any lens can be used to take a portrait of someone. However, the results can be very different depending on its focal length.

The distance required between you and your subject, when you take a head and shoulders portrait, depends on the focal length of the lens. If you have a wide angle lens then you need to stand very close to them in order to have their head and shoulders fill the frame. But if you have a telephoto lens then you need to stand a fair distance away from them for the same effect.

This distance results in a change in perspective. Try this experiment with someone in real life without using a camera. If you stand really close to the person and look at their face you’ll notice that this position tends to emphasize their nose and make their forehead look like it’s sort of sloping away. But if you’re further away from them then their face tends to look flatter. And generally speaking people find that a slightly flatter perspective on a face is usually a bit more flattering in general. Portraits taken with wide-angle lenses can, in fact, have a rather comical or grotesque effect.

So generally photographers like to use lenses of about 85mm to 135mm in length when taking head and shoulders portraits, depending on the look they’re trying to achieve. Some fashion photographers even use 200mm and 300mm telephotos for a particularly flat effect. You can take photos of people with 50mm and shorter lenses, but these lenses tend to distort the face somewhat. Such shorter focal lengths are, however, perfectly fine for waist-up or full body shots.

Canon make a number of of popular lenses used for portrait photography. Two of the more affordable ones include the compact and sharp 85mm 1.8 and the 135mm 2.8 SF which has a “soft focus” feature which allows you to introduce image-softening at will.

Finally, note that the lens focal lengths I list here are for 35mm film or full-frame EOS SLR. If you’re using a camera with a smaller image area - digital or APS - then the ideal focal lengths for portraits are going to be shorter. For example, a 50mm lens is generally considered a bit short for most portraiture with 35mm film, but when mounted on an EOS 10D digital camera it takes photos much like those taken with an 80mm lens on a 35mm film camera.