Jade16 said:
why are longer focal lengths preferred?
Well, for a number of reasons. I'll simplify: if the camera is too close to a person, say inside of 7 feet, then the nose and anything that extends toward the camera tends to become literally larger on-film than it ought to be: extended hands and arms are made larger, the nose is made larger, the ears can tend to be a bit larger, feet and legs that are extended toward the camera look larger. In other words, to get a big, frame-filling image with a short length lens, there is a terrific temptation to get too close, and being too close causes exaggerated size to things that are closer to the camera, such as the nose. Long lenses tend to force the camera to be placed back a ways!
Second, a longer length lens has a narrower angle of view, so it's easier to select the subject, and to eliminate other things from the environment. The longer length lens brings with them something called selectivity or
narrow angle of view; which allows you to eliminate a wide swath of the real world stuff that would be seen in the picture in the background if a short lens length were used.
Third, the longer lens tends to physically (meaning LITERALLY, in real,measurable with a ruler type terms) enlarge background elements, and to make the degree of background blur higher than with a shorter lens. The longer lenses, like 200mm and 300mm cause a very pronounced magnifying of background elements, and they "compress" the planes of the face, which can look quite attractive. The nice blur the longer lengths give allows the backdrop to be subdued. This is why the 70-200mm zoom and to a lesser extent the 70-300mm lenses are handy for portraiture outdoors. A long telephoto magnifies the background's size, and can make a distant background appear to be very close to the subject.