In photography, 85mm lens is recognized as special lens for portrait photo of face. Why it's 85mm? what about the others? 24mm or 200mm? Based on many expert portrait photographer which is have many experiences to shoot thousands human face when using lens 85mm, the face of model will looks so proportional and very interesting.

The wider lens will make the face looks more convex, distortion nose, and the chick bones looks more chubby. Tele lens will make the face is so flat, thinner than it should be. 

To get you more understand, look at the changes of the face shape illustration bellow! There is a model face that was shot by different lenses which is have each focal length, they are 20mm, 2mm, 28mm, 35mm, 50mm, 85mm and 200mm .

For additional notes, 50mm lens actually is good enough for portrait lens. But 85mm lens give more advantages of background isolation which is more good , because 85mm lens give you more fine depth of field (DOF)  better than when you using 50 mm lens, so the bokeh is more good. The wide lens like 35 mm or wider is also used for portrait photo when  the background is matter! its not for tight portrait which is full frame of face, just look at this sample pic bellow : 

Of course the focal length that we talk about is in full frame censor. When you use APSC camera or moreover the four-third micro, the focal length will follow the crop factor law.