About perspective and focal length
A new group for this: Ipernity Academy.
I've seen that there is interest for these articles, but perhaps they're too long. So I start a series of shorter ones (translators welcomed!).
I talked about the effect of the focal on the quantity of light hitting the sensor with a zoom lens, but there is another more important effect: perspective distortion.
Simply said: when you change the focal length of you camera acting on the zoom, you not only magnify the focused subject, but you also change the appearence of the background. If you change your distance from the subject to have it of the same size on the image, you'll have a different background.
Beware of the "background" word: if you're making a close portrait, the eyes are the "background" of the nose; the wider the lens angle (less millimeters as in wide-angle, fisheye, macro, ...), the larger will seem a face in a portrait; the tigther the lens angle (more millimeters, longer focals, zoom, tele, ...), the narrower will seem the face.
You can use those notions for artistic effects and trompe-l'oeil. Anyway you must account for it when you move closer/farter from a subject: if you make a portrait with a close-up, you'll magnify the nose of the subject and the face will seem larger than it is and sometimes the portrayed person will be a little bit upset from this effect...
For a more thorough introduction: www.ephotozine.com/article/Focal-lengths-in-portraits