Seven considerations when buying glass
Apart from price, there are seven main considerations to maken when buying a lens. Because every photographer has different needs, it is difficult to give advice if you don't know a person's "consideration mix". With help of this text , it's easier to describe your requirements, ask for advice, and interpret another person's response. (I've compiled this list because I see a lot of discussion going on about this topic and felt the need for a "framework" - might also be a personal problem of mine 
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1) How "fast" does it need to be?
a) In low light situations, you want a lens with large maximal aperture (low f-value). Fast lenses are considered anything of f/2.8 and below, the lower, the better. That means, all other things equal, you can take pictures with faster shutter speeds with a fast lens than with a slow lens. This is mainly important if you photograph moving subjects in relatively low light.
b) some lenses, while optically not that "fast", have an image stabilising element that make it appear faster in some situations. Specifically, these lenses help to prevent camera shake during longer exposures. As one photographer put it, it's like having an invisible tripod. The downside: IS is expensive, makes the lens heavier and bigger, and drains battery somewhat faster than other lenses.
c) With a fast lens, you can add a teleconvertor en retain autofocus. In normal light situations, lenses will focus if aperture is f/5.6 or better. Add a 2x teleconvertor and you lose 2 "f-stops", converting your expensive f/4 L lens to a f/8 lens that will not autofocus. A still more expensive f/2.8 lens will retain its autofocus possibilities, though behaving as an f/5.6 lens.
d) focus in low light: lenses focus with their diaphragm blades opened. Therefore, "fast" lenses will focus in low light, "slow" lenses won't (even when using a flash, except when the flash has focus assist)
e) Background blur: generally, the faster the lens, the better the background blur, the nicer the background of a subject will show, the better the subject will stand out from the background.
f) bokeh (how "nice" the background blur is): subjective, of course, so take a look at examples when this is important to you.
2) Do you need it to be equipped with zoom?
This depends on your expected use, the "crop factor" of your camera, and the other lenses you have and are willing to carry with you. Zoom lenses are more versatile but less sharp than primes and also bulkier and more expensive. Generally, depending on your main subject, you'll want these lenses:
a) walkabout lens: medium range-zoom from wide-angle to medium-tele, to cover most situations you are likely to encounter
b) landscapes: wide-angle to get broad panoramas and large depth of field
c) studio portraits: primes are the choice of professionals, because you have the photographer-subject distance under control
d) outdoor portraits: medium-range tele lenses are best suited, so that you can achieve nice bokeh and also move a little away from the subject, so that (s)he does not become upset with a lens in the face.
e) animals or sports: medium-range to super tele, zoom or prime, if possible fast because the subject will move fast.
f) macro: tele, prime or zoom, will give better magnification. (note: focussing distance is also important here, see below)
3) What size is acceptable?
a) light: is it easy to carry with you in a bag? around your neck? does your tripod hold it or do you need to buy and carry a heavier tripod? How easy is it to keep the lens steady, to prevent camera shake, during your photo shoots?
b) small: is it easy to carry with you, and does it draw attention to you?
c) filter size: do you need specific filters, such as a neutral density filter, for each of your lenses?
4) Will it attract (un)wanted attention?
a) white: if you take out your Canon 70-200 L, people will think you're a pro and get out of the way/become impressed/come to ask questions about photography/run to you with a revolver and take the lens away from you. A similar effect, though more subtle, is achieved with any lens that has a red line around its barrel.
b) lenses with a hood look more professional than lenses without. The hood is actually useful, to prevent stray light falling into the camera, but its most visible effect may be people's reactions.
This factor is sometimes called the WOW-factor of the lens. But it can work in two directions: sometimes you might want to impress people with your gear, sometimes you might not. That depends on the situation and on yourself. It's just that you can keep this in mind when choosing your new lens.
5) How does it focus?
a) manually: even if you don't use manual focus today, when you really want to learn something about photography, you will need manual focus from time to time. If you need to switch between manual and autofocus frequently and while taking pictures, you want to do this blind and not inadvertently. Also, pro lenses allow you to autofocus, then adjust the focus manually without leaving autofocus mode.
b) auto-focus: is it quick, accurate, and silent? Focus speed is important so you can catch the right moment. Accurateness is a must, you don't want backfocus or frontfocus effects because that ruins almost all photos and makes autofocus unusable. Silence is important when photographing animals or suspicious people and is generally not much of a concern. At present, Canons USM may be the best autofocus technology since it is fast, accurate and almost not audible.
c) minimum focussing distance: the closer you can get to the subject while retaining focus, the higher magnifications you can get. Tamron lenses are known for having close focussing, though you can buy extension tubes for other lenses to shorten this distance too.
6) What optical quality is needed?
a) sharp: sharpness is best measured when the lens is fully open (i.e. at max aperture) and at max tele. Consumer lenses will go soft in both cases, pro lenses remain sharp. Primes are generally sharper than zooms.
b) vivid colors: depending on the glass used inside the lens, the quality of the image on the sensor of your camera body can be very different. Pro lenses are known for their accurate and vivid colors.
c) no chromatic aberration, such as "purple fringe" around leaves with a blue sky background.
d) no geometrical effects (barrrel distortion at the wide angle end/pincushion distortion at the tele end). Unless you need the awkward effect of fisheye lenses, that is. But then, you'll probably buy the lens for that specific purpose.
7) Is it generally well-built?
a) sturdy: does it feel like you can tear it apart just by zooming or focussing? Not a big issue, since most lenses won't survive a fall from let's say a hip height. So that does not influence most photographer's decisions anyway. If you want your lens to stay in optimal condition, don't drop it.
b) weather-resistant: does it have a sealing that prevents water (rain, action photo on the water) from entering the lens and the body? Should only be a concern if your camera body is water resistant somehow.
c) does it have internal zoom to prevent attracting dust while zooming?
d) does it have internal focus, so that the hood won't turn when focussing?
e) ease of use when zooming and focussing: does this happen in a smooth fashion? Fast enough but not too fast?
Notes:
- these seven considerations are just one side of the balance (the other side is the price).
- the order of these considerations doesn't have any specific meaning. In fact, you should try to order these considerations for yourself when buying glass.
PeteR.
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Guess that's what adds flavor to the forum....
