You've probably heard before that you should not shoot portraits with a wide angle lens because it distorts the features in an unflattering manner, but there's more to that concept than just that advice. This great video will quickly clear up what is really happening when you change focal lengths.
Coming to you from David Bergman with Adorama TV, this helpful video will show you why you can actually shoot portraits with wider lenses without fearing major distortion if you put some thought into it. Bergman has a good way of thinking about it: consider the relative distance. Filling the frame with your subject when using a wide angle lens requires you to get very close, and as such, the relative distance between features is much more exaggerated than if you were standing at the required distance to fill the frame with something like an 85mm. On the same token, as you continue to use longer focal lengths and thus have to step farther back to keep your subject the same size in the frame, the relative distance between features shrinks, and thus, the features appear flatter. Check out the video above for a great demonstration and rundown of the concept.