Possible Duplicate:
Which focal-length lens is usually used for portrait photography, and why?
Assuming a similar aperture, what difference would you see using a 50mm, 85mm, or telephoto lens for a portrait?
Would the backgrounds appear differently? For example, It's not clear to my why someone would get a 50 and 85mm prime lens instead of just say a 50mm and move closer?
Answer
It's all about foreshortening, the effect by which the depth of the scene appears compressed. Different focal lengths just permit you to be different distances from your subject and still give the appropriate framing.
Subject distance is the key value here. If you are a kilometre away from your subject, then the tip of their nose is a kilometre away, as are their ears. If you are 10cm away from your subject then the tip of their nose might be 5cm away and their ears 15cm. These distances are suddenly important as the nose is three times closer it will appear three times larger.
Thus if you use your 50mm lens and simply get closer as you suggest, then you will enlarge relatively those features that are closest to the camera as well as accentuating any affects of the subject not being parallel to the camera (e.g. if they are leaning forward their forehead will be enlarged relative to their chin).
Beyond a certain distance the difference in depth between features become small enough that you cease to notice it. For this reason photographers tend to settle on a focal length that is long enough to prevent odd foreshortening effects, but not so long that you need a walkie-talkie to communicate with your subject. For APS-C cameras this is about 85mm.
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