Many times I see threads about 'what focal length' and both the question and the reply are couched in non-specifics about the frame size for which the comment is made. Remember, FRAME SIZE DICTATES the context in which a given focal length will behave. To borrow from another thread this point: 28mm is not 'wide' when it is used on a APS-C format dSLR like 350 or 30D
'Normal' is typically defined by the diagonal measure of the film frame. A 'true normal' for 35mm film (or 5D and 1DsII) is 43mm! But the industry has long settled on the convention that 50mm (or thereabouts) is 'normal' for that size frame. On that frame, 35mm is kind of 'in between'...semi normal and semi wide angle. 28mm is generally considered a true 'wide' , and 24mm is 'very wide' and 20mm is 'super wide'. At the longish end of things, 70mm-100mm are 'short tele' and '135-200' are 'tele' and 300mm and longer are 'long tele' and 'super tele' by convention.
In the world of portraiture, the shooting distance from the subject defines the facial perspective that we generally find most pleasing and 'realistic'. The focal length for a given shooting distance defines the 'framing' of the person and how fully they fill the frame area. Convention has found that about
70-90mm is good for waist-up portraits,
90-110mm is good for head and shoulder portraits, and
120-150 is good for headshots (a modelling and acting type of shot for sending out to casting people). Of course, different photographers will tell you that THEY do not follow these conventions, but it is generally true that most professionals and amateurs in past decades with 35mm film have found these ranges to be valid ones -- until you discover your own style and preferences!
Reminder: All of the above statements are in the context of 35mm film cameras. For APS-C dSLR, divide the above by 1.6 in order to find the equivent for your camera. So 28-30mm would fall smack into the 'normal' range, 17mm is a 'wide' but is not 'very wide'. You need a 15mm to get into 'very wide' and a 12mm to get into 'super wide'.
So for portraiture for APS-C format camera, the numbers mentioned two paragraphs ago would translate to:
about 50-60mm is good for waist-up portraits,
60-70mm is good for head and shoulder portraits, and
80-90 is good for headshots (a modelling and acting type of shot for sending out to casting people).
So the next time YOU 'recommend' a particular focal length, it is important to state the frame of reference that YOU are making the statement in (APS-C or FF, for example) and also to take into consideration the camera that might be listed in the OP signature line. And equally important, if you are the OP member, tell us which camera you are asking about! With the advent of FF digital, and with novices having the financial ability to 'buy the best' regardless of their level of skill, we cannot assume APS-C format is what they are referring to, either!
