Each of these portrait lenses will merit a blog unto itself, but let’s start with the general principles of the portrait lens.
First off, people prefer to end up looking human in their photographs. Using wide angle lenses for portraits does not generally pan out, since they distort the face unnaturally. Focal lengths of 35 mm or below tend to be problematic, and really wide lenses are somewhat comical in portraits. See the photo to the right to get the distortion picture. Ideally, portraits are taken with focal lengths of 50 mm – 200 mm. You will have to stand closer to the subject with a 50 mm lens than with a 200 mm, so consider how comfortable your subject is with the camera near the face.
50 mm is the “normal” lens, and has least distortion. Higher focal lengths will compress and flatten the image; the 85 mm lens is considered magical for “distortion” that is particularly flattering for the human face. The 70 – 200 mm zoom is the most popular portrait lens, since the zoom affords easy in the camera framing of the image.
Each of these portrait lenses is preferable if they are “fast”. A fast lens is one with a wide aperture, able to let in a lot of light. Fast lenses have more glass and more weight. A big lens captures more light. If you can afford the cost, the 50 mm f1.2 and the 85 mm f 1.2 are the ultimate in fast lenses. These will let you open wide and take photographs in low light without a flash. When the lens is wide open, the depth of field, the amount you see in focus, is diminished. At f1.2, the nose may be in focus, but the ears out of focus, so wide open the photographer will need to plan the shot. Shallow depth of field may be used to artistic advantage.
The fastest available lens for the 70 – 200 mm zoom is f2.8. The f2.8 aperture is wide enough to make the subject sharp, but the background pleasantly blurred (referred to as bokeh). These portrait lenses are expensive, and I would only purchase them if one also had a professional level camera body. Why put a $2500 lens on a cheap body? That makes no more sense than a Ferrari engine in a Subaru body.
Since Nikon has yet to make an autofocus 50 mm 1.2, this is a justification to prefer Canon. You’ll do OK with the Nikkor 1.4 and it is less costly, but that big Canon glass delivers (Nikon lenses are under the name Nikkor).
So if you can justify the expense, the Canon 50 mm f1.2, the 85 mm f1.2 and the latest version of the Canon image stabilized 70 – 200 f2.8 are the cream of the crop for the mainstream lenses. Beware of Internet merchants offering much less expensive Canon lenses in the 70 – 200 size because they will very likely be an out of date version that is nowhere nearly as good or lack image stabilization.
You’ll get good images using the second grade level of lenses in Canon, Nikkor, Sony, or by using Tamron, an aftermarket lens. Just stick to the 50 – 200 focal length and get the best lens you can realistically justify. The key in portraits is to have a lens so you can take head shots with a focal length that is not going to distort the image in a bad way. Then it is about light, glass quality, camera quality, processing skills and perhaps most of all, capturing the emotion of the moment.