cloudy said:
I was just wondering why there was such a variety of types
Ahh, now that I've found the right end of the stick....
If you take a look at the Canon EOS lens range then the lenses can be split into a few groups
L Series - These are the pro quality lenses which include Fluorite or UD elements to improve sharpness and reduce colour fringing. While there are a few L zooms available the bulk are fixed focal length and cover from the ultra wide 14mm to the super telephoto 1200mm f5.6 monster.
DO Lenses - Only a couple exist at the moment, they use a diffractive element similar to those reversing lens things you see in some cars to reduce the size and weight of the lens. The 400mm DO is about 2/3 of the weight and size of the equivalent L series lens.
Others - The rest of the range doesn't have a designation as such but that's not to say that they're no use, the 50mm f1.8 & f1.4 lenses are sharper than most of the L series zooms because they're less complex. There are some mutts in the range though but these are the older lenses which are being superceded by new models.
Within each series of lenses there are different focal lengths etc which are used for different purposes.
Wide angle (< 50mm) - as the name suggests you get a wide field of view, normally for landscapes etc but super wides & fisheyes give some great perspective effects. This can also be a downside if you want to avoid barrel distortion - the more expensive lenses tend to have less distortion. Normally wide angles are fixed focal length but recently there has been a spate of new super wide zooms which have overcome a lot of the inherrent issues.
Standard lenses (~50mm) - These lenses give a perspective view which is very similar to the human eye and hence were traditionally the usual lens supplied with a new camera. Very simple to make and usually very good lenses
Short telephoto (< 200mm) - These cover a huge range of disciplines, close up sports like boxing or basketball, portraiture and macro work all tend to have lenses in this sort of area.
Long telephoto (>200mm) - This is the sole preserve of the wildlife and sports guys. Long focal lengths mean large lenses and precision optics, hence the lack of cheap options here.
OK, so that's qualities and types but there's yet another thing to consider - maximum aperture.
A lens which is capable of f2.8 will allow for a shutter speed 4 times greater than one which can only do f5.6 in a given situation. This is important in low light or if a fast shutter is required to freeze action. Now the obvious question is then "why not make all lenses f2.8 or better yet f1.0 lenses?" The answer is size - the f number is the ratio of the diameter of the aperture iris to the focal length of the lens and hence the diameter of the main lens elements. For example the Canon 300mm f2.8L has a front element which is over 120mm in diameter - this is both expensive and heavy. By sacrificing a stop of maximum aperture and going for an f4 lens allows the diameter to come down to 77mm and the price to drop to a little over a quarter of the f2.8's price tag.
Anyhoo, let's pause the ramble there without going into zoom v prime, macro, tilt shift, image stabalisers etc. Let me know if you want detail on those.
I was down at Goodwood the other weekend and there were a couple of times when I had to dash across the paddocks to get the shot I wanted, when you're charging through a crowd with a big camera and a big piece of glass on the front folk tend to get out your way 
