Avoid distortion when taking portraits using wide angle lenses
Generally speaking it's best to avoid using a wide angle lens or the wide end of the focal range, otherwise you could end up with distorted portraits. Assuming you're using a prosumer DSLR camera, it's best to stick to the 35mm -> 80mm range.
The magnification of a 35mm lens closely replicates what we see looking through our own eyes.However, most portrait photographers prefer to use 50mm or 85mm lenses, this allows them to stand further from the subject and so slightly flatten the perspective.
This is not to say wide angle lenses cannot be used for portraiture. The unique properties of wide angle lenses can be exploited to create eye-catching and dynamic portraits that stand out among more conventional ones.