Because photography is an art form, you could argue that there is no right or wrong focal lengths for taking portraits. While I agree that the best focal length is the one that gives you the results you desire, there are focal lengths that are generally more flattering to the human form than others. We will discuss wide angle, medium, and telephoto focal lengths and the pros and cons of each when photographing portraits.
Generally, a wide angle lens is not the first one I’ll reach for when shooting a portrait. Anything in the 10-25mm focal length on a full frame body tends to distort the subject the closer they are to the lens. A wide angle will exaggerate prominent features and make them appear even larger. It’s definitely not the most flattering focal length, especially if you are shooting close to your subject.
A wide angle lens can be essential though if you are looking to capture someone in their environment. Think about how much more compelling your photo of a F-16 fighter pilot would be if you captured him inside the cockpit of his jet and the image was wide enough that you could see the canopy around him and the some of the controls. Sometime a wider focal length will help tell a better story because it gives context and location to the subject.