Fixed focal length (prime) vs. zoom lenses
Zoom lenses first appeared in the sixties and have been becoming increasingly popular ever since then. As of now, they by far outsell fixed–focal–length (FFL) lenses and the majority of new 35mm DSLR cameras come with zoom lenses. In the early stages there was a significant image quality gap between FFL and zoom lenses but with the help of the computer–assisted lens design as well as modern multi–coatings, this gap is far narrower now and some zoom lenses are truly outstanding in terms of optical performance. In theory, zoom lenses might completely replace FFL lenses at some point; in practice, however, this is unlikely to happen in the foreseeable future—let us see why.
Convenience and flexibility go a long way
The main advantage of zoom lenses is incorporation of a continuous range of focal lengths in one lens, which transforms into flexibility and convenience. One zoom lens can replace a number of FFL lenses in terms of focal lengths and allows to quickly and conveniently change focal length settings. This saves time on changing lenses and is important for shooting events or action; it also allows for exact framing when movement is restricted. However, this convenience comes at a cost. Pro–level zoom lenses that can deliver image quality on par with that of FFL lenses are very bulky, expensive and heavy (their cost and weight often are greater than the total cost and weight of the several FFL lenses that they replace!); thus, the very point of their convenience is somewhat jeopardized. As to the "prosumer" and "consumer" zoom lenses that are not as bulky and reasonably priced, their overall optical quality, unfortunately, remains inferior to the quality of FFL lenses. By and large, it is true to say that some zoom lenses are very good and a lot of them are quite bad, while some FFL lenses are not so good but most of them are very fine.
Several paragraphs below deal with the disadvantages of zoom lenses and show that FFL lenses are still better in many respects (generally, they are better optically, smaller, lighter, faster and cheaper). Yet, once again, zoom lenses are much more popular, which goes to say that convenience and flexibility go a long way.
Not as fast...
As a rule, FFL lenses are considerably faster—the best pro–level zooms have a constant maximum aperture of f/2.8, which is as good as it gets with zooms; in case of FFL lenses, on the other hand, f/2.8 is considered rather average if not exactly slow, and many popular focal lengths have f/1.8 and f/1.4 designs. Some FFL lenses even go up to f/1, albeit at the cost of bulkiness and very high price.
...and even slower
In some cases, if a FFL lens and a zoom lens at the same focal length setting have the same maximum aperture, the FFL lens might actually be faster than the zoom lens. This might happen due to the fact that f–stops are about geometry, i.e. about the geometric relationship between the focal length and the size of the diaphragm opening, while actual light transmission values are measured in t–stops (they are not shown on most lenses)—and two lenses do not necessarily transmit the same amount of light at the same f–stop! In other words, all lenses have the same f–stops, but at a given aperture they might transmit different amount of light due to the light transmission losses, which result from lens internal light reflections and absorptions. Obviously, the more elements a lens has, the more light is lost in transmission and the slower the lens becomes at any given f–stop. With the FFL lenses the difference between f and t stops is practically negligible because they normally have a relatively small number of optical elements and not much light is lost in transmission. In zoom lenses, however, which normally have at least twice as many elements as FFL lenses do, the difference might be 1/3 stops or even up to 2/3 stops. In this case, not only is your lens slower than you think, but this might also present a problem if you use a hand–held meter as all your images will be underexposed. With built–in TTL metering, however, this is accounted for automatically and, due to this, you would not even be aware that your zoom lens is actually slower than you think.
If you look at the issue from a different perspective and set a FFL and a zoom lens so that they transmit the same amount of light, the zoom lens will end up being used at a wider aperture to compensate for light lost in transmission and thus produce a shallower depth of field.
You can easily check this effect through the following procedure. First, mount an FFL lens and determine exposure for a given subject area under certain lighting conditions using your camera's built–in TTL meter in shutter–priority mode; take note of the apperture suggested by the camera. Then mount a zoom lens and determine exposure for the same subject area under the same lighting conditions with the same pre–set shutter speed. In the latter case, does the meter suggest using the same f–stop? If yes, that means that the zoom does not have significant light transmission losses; if no, i.e. if the meter indicates a longer exposure, the lens transmits less light than the FFL lens (the difference in f–stop readings is that variation) and the meter of your camera automatically compensates for it by imposing a longer exposure.
What is with the variable maximum aperture?
It is only pro–level zoom lenses that have a constant maximum aperture at all focal lengths—most other zoom lenses have a variable maximum aperture (usually in the range of f/2.8 through f/5.6). This has two disadvantages. First, it simply is very inconvenient as you never know what the maximum aperture at any given focal length is. I mostly shoot in aperture–priority mode and the lack of constant maximum aperture is really frustrating to me. Second, if the maximum aperture at a given focal length is slower than f/2.8 then the viewfinder view will not be as bright and autofocus performance might be influenced, too.
More elements—more flare and distortion
FFL lenses normally have quite fewer elements and, due to this, have much less flare and are easier to correct for distortion. Zoom lenses, on the other hand, normally have more than a dozen elements and, of course, are much more prone to flare and ghosting. Also, distortion is more difficult to correct as zoom lens designs require a shifting position for the diaphragm. In fact, these two problems, which might be insignificant in certain types of photography, present bigger problems in zoom lenses than even sharpness—in expensive designs a high degree of sharpness is achievable but flare and distortion are more difficult to control. For example, even the pro–level AF Zoom–Nikkor ED 80–200mm f/2.8D exhibits very strong pincushion distortion at 200mm.
Seeing the world differently
All of the above are mainly technical considerations. They, of course, are significant but, in my mind, it is of a far greater importance that FFL and zoom lenses impose quite different shooting approaches—they make the photographer have a very different perspective on things and perception of the world.
Zoom lenses are wonderful tools if used properly. Many photographers, however, become victims of convenience and misuse them. Apart from exact framing under restricted movement, the whole point of using different focal lengths is perspective control. That means that one should first decide what perspective effect he wants to achieve and then choose a corresponding focal length and shooting position. When using zoom lenses, however, many forget about perspective effects and zoom in and out to just conveniently frame an image.
In my experience, unless you have a very strict attitude towards using zoom lenses, they might impose a mindless approach towards seeing things and composition—they make you look at a scene before your eyes and then zoom in and out to see if anything good comes out of it. You kind of tend to lazily observe if the zoom lens can make something out of a scene before you have actually seen anything in it.
FFL lenses, on the other hand, predispose me to see and not just look, they make me make an effort to first discover something before my eyes and only then look at it through the camera's viewfinder. With time you learn to see the world as each focal length does and each time you mount a different focal length lens on your camera, your mind switches into a different perceptive mode. On certain occasions this allows me to have only one FFL lens with me and feel no anxiety about not being able to use lots of different focal lengths. It also allows me to comprehend how people could produce very fine photography in the sixties, the fifties, and even earlier. I personally find all this very stimulating and rewarding.
So, what to choose—FFL or zoom lenses? This question has to be answered by each photographer depending on his style of photography and considering all the factors outlined above. It is important to keep in mind, though, that FFL and zoom lenses are not entirely contradictory—in many cases they are supplementary and one might use both for different purposes—as they say, horses for courses. As far as I am concerned, I use zoom lenses with my DSLRs when speed of shooting and convenience are importan; for contemplative and meticulous art work I use medium format film with FFL lenses.