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.
When using a wide angle lens up close, objects closer to the lens will appear relatively bigger than they actually are, so for example the nose will appear to be larger, while the ears and sometimes the eyes will appear smaller.
These distortions mean wide angle lenses are inappropriate for most serious portrait studies. However, there are certain types of portraits that can use the distortion to great effect.
My son Adam (he doesn't really have a huge head!)
For instance, the caricature effect created by a wide angle lens can be quite humorous, and for this reason it is not uncommon to see wide angle portraits of comedians. If you are planning to take a wide angle portrait it is important to make the effect look as deliberate as possible, otherwise it will simply seem as if you have made a mistake. One way to do this is to have the subject pull an odd or exaggerated facial expression that complements the caricature effect that a wide angle lens creates.