Recently I decided to demonstrate the difference a lens choice can make in a portrait photograph. Unfortunately for her, I persuaded one of my daughters to be the test subject. I started with a 200mm telephoto lens and filled the frame with her face. A very basic head shot.
Then I did the same thing with a whole series of lens focal lengths: 150mm, 100mm, 70mm, 50mm, 35mm, 28mm, 24mm, 20mm, all the way down to a very wide angle — 14mm! Each time I got closer and closer so I filled the frame the same way I had on the first shot. This meant starting off all the way across a large room and ending up (literally) in her face.
I’ll let you decide which gives the most flattering look. (Your choice may not be the one that best reproduces what she really looks like.) 
While my choice is admittedly subjective, the fourth photo from the left in the top row looks the most like the real woman. It was taken with a 70mm lens. Most portrait photographers like the look they get from lenses in the 70-110mm range. They feel it gives them a flattering and realistic representation of the subject.
Longer telephotos tend to flatten the face (see the top left photo) just like they do with landscape images where great distances between places are compressed by telephotos. Wide angle lenses tend to exaggerate the spatial differences (see the bottom right). That’s why she has the typical “big dog nose” look in the last shot.
I suspect that shot will cost me a few points off my Father’s Day present this year. She may even forget the date. This phenomenon can sometimes be helpful with your subjects. A little longer lens can be more flattering to a subject with a large nose or chin. A wider angle can give a little more dimension to a flat face.
Of course, you want to stay within the bounds of realism. When I do portraits, I usually use a zoom that lets me take shots in the “sweet spot” range of 70-110mm. After I’ve picked the proofs to show, I let the client decide which one they prefer.
[Editor’s Note: Learn more about how you can fund your travels and make an extra income with photography, travel writing, blogging, and more in our free online newsletter The Right Way to Travel. Sign up here today and we’ll send you a new report, Five Fun Ways To Get Paid To Travel: A Quick-Start Guide, completely FREE.]