From a technical standpoint, any lens from 8mm to 800mm can be used to photograph people, meaning that every lens could be classified as a "portrait lens." But just because a lens can be used as a portrait lens does not necessarily mean that it should be utilized as such for general portraiture. There are two primary characteristics which make some lenses ideally suited for capturing flattering images of people. Let's explore those characteristics in more detail to determine why they're so beneficial to portraiture.
Medium Telephoto Focal Length (or Focal Length Range)
Typically speaking, lenses with a medium telephoto focal length (or focal length range) tend to work best for general portraiture. While the ideal range can vary with personal preference, generally speaking a lens featuring a focal length in the 85 - 135mm range on a full-frame camera (55 - 85mm on a 1.6x crop sensor camera) will provide a compressed view which mitigates or eliminates the foils of perspective distortion.
Why is perspective distortion a problem in portrait photography? Because when using a relatively wide focal length, combined with a close working distance, facial features nearest to the camera – typically noses – become enlarged in the frame.
Understandably, few subjects appreciate the look of a disproportionately large, prominent nose in their portraits. A longer focal length is the key to keeping facial features flatteringly proportional. Of course, shorter focal lengths (35 - 50mm, especially) can be used for more loosely framed subjects where the background becomes a more meaningful compositional element in the image, just as longer focal lengths can be employed for increased working distances, narrower angles of view and even flatter perspectives. However, it's tough to go wrong with a focal length in the 85 - 135mm range when general portraiture is your primary agenda.
Another key element prized by portrait photographers is a very wide aperture – in the f/1.2 to f/2.8 range – allowing for subject isolation via shallow depth of field (DOF). Of course, the wider the aperture, the shallower the DOF. With background elements unrecognizably blurred, distractions can be eliminated and viewers' eyes can be drawn precisely to the subject.
However, a shallow depth of field isn't the only benefit that a very wide aperture brings to the table as far portraiture is concerned. Another big benefit of wide aperture use is that faster shutter speeds can be utilized to freeze subject motion thereby allowing sharp details to be captured.
Portrait Lens Recommendations
I considered listing several lens recommendations in this section, but really there's no need. We already have that covered in our Portrait Lens Recommendations section (at least as far as Canon shooters are concerned). For Nikon shooters, check out the Portrait Recommendations and substitute a comparably-spec'd Nikon (or Nikon-mount) lens.
As I mentioned at the beginning of this article, any lens can be used to record photography's most important subjects, people. However, lenses with a medium telephoto focal length (or zoom lenses that encompass such focal lengths) coupled with a wide aperture can aid in capturing flattering imagery that can make your friends, family, or clients look their best.