A question most Canon lens buyers face - Should I buy a zoom lens or one or more fixed focal length (prime) lenses?
Canon makes both for a reason - and there are good reasons to have both. I own and regularly use both (but use zoom lenses much more).
This article contains many generalizations regarding the differences between Canon's zoom and fixed focal length lenses. There are often specific examples that disprove certain generalizations, so be sure to read the specific Canon lens review for the lens(es) you are considering.
Also note that my generalizations should also be taken comparatively within Canon's lens series. Canon L Series zoom and fixed focal length lenses should be compared with each other, and Canon's consumer series zoom and fixed focal length lenses should be compared with each other.
I should also note that some of Canon's consumer fixed focal length lenses can equal or better some of Canon's L Series zoom lenses for sharpness at some settings. Sharpness is not everything, but it makes a big difference in image quality. Again, check the specific lens reviews before making your decision.
The major advantage a zoom lens has is its versatility. A photographer using a zoom lens can quickly and properly frame and capture a fleeting opportunity. They can also capture many different framings of the same subject within seconds. The fixed focal length lens user may still be sneaker-zooming (which also changes perspective) to the right distance when the subject disappears - or is no longer in that cute pose. A subject with a rapidly changing distance also favors a zoom lens.
The fixed focal length lens user may alternatively shoot with the framing they are given. This results in a degraded picture - The photographer is forced to crop the picture in postprocessing or leave potentially important details outside of the frame. This of course reduces the image quality advantage just discussed.
Zoom lenses require fewer sensor-dust-collecting lens changes - and fewer lenses to carry. Lens changes are inconvenient and take time - and provide opportunity for dust to get to the sensor. On the other hand, some zoom lenses move air in/out and can be a source of dust themselves.
In recent years, the general public has heavily embraced zoom lenses. This is reflected in the type of new lenses Canon has introduced. Since 1999 (when Canon refreshed its Super Telephoto lens line), zoom lenses have been introduced much more frequently than fixed focal length lenses.
Canon fixed focal length lenses are usually faster (have a wider aperture) than their zoom counterparts. To me personally, this is the greatest value of a fixed focal length lens.
A fast fixed focal length lens allows action-stopping shutter speeds to be used in low-light situations. Fixed focal length lenses are sometimes the only solution for dark indoor sports photography. I should mention that the ever-improving high ISO performance of new DSLRs is making fixed focal length lenses less important in these situations.
A fast lens also allows excellent subject isolation by blurring the distracting background. The results can be beautiful. This benefit is a strong one.
Conversely, an extremely wide aperture creates a razor-thin depth of field - this is not adequate in many situations.
Canon fixed focal length lenses are smaller than their zoom equivalents. Canon non-L fixed focal length lenses are generally much smaller than Canon L-zooms in the similar focal length range. But you might need to carry several prime lenses to cover your needs.
Canon fixed focal length lenses are usually sharper than their zoom counterparts - especially in the corners, and much sharper in the non-L lenses. This difference is much less noticeable in the Canon's L Lens Series, although some non-L fixed focal length lenses can best L-zooms slightly at comparable apertures in this regard. Fewer lens elements along with a reduced tendancy for contrast-robbing flare contributes to this attribute.
A very rough generalization: Canon fixed focal length lenses are usually less expensive than their zoom counterparts - if they have comparable wide apertures. When the fixed focal length lens apertures start getting much wider than the comparable zoom's aperture, prices may actually cross back over. Canon's non-L fixed focal length lenses are always much cheaper than Canon's L fixed focal length lenses and zooms.
Any cost savings can evaporate when multiple fixed focal length lenses are required to cover the focal length range of one zoom, but having backup lenses available is a nice side effect of multiple fixed focal length lenses.
Some focal lengths are only available from Canon in a fixed focal length lens - such as the Canon EF 500mm f/4L IS II USM Lens or the Canon EF 14mm f/2.8 L II USM Lens. Some lens types are not offered in a zoom - Canon does not offer a true 1:1 Macro zoom lens.
I have heard photographers talk about using strictly one fixed focal length lens to spur creativity. I don't agree with that reasoning as you can just as easily spend a day with your zoom lens set to a single focal length. Just use a little non-marking tape to fix your zoom lens at that focal length. Then spend the next day at a different focal length if the basic premise helps you.
Of course, when the shot-of-a-lifetime happens upon you, you can break your promise to yourself and zoom to the proper focal length to capture it! Some also say that all images start looking the same when shot with a single prime lens. You get the same perspective with the same framing.
Some photographers only need one focal length for most of their work. If this is the case for you, a fixed focal length lens likely makes the most sense. Pick the one that is best for you.
My personal preference is for canon's L-zooms. I will usually pick up an L zoom in the 24-200mm range over my prime lenses UNLESS I need a higher shutter speed than my zooms' apertures can give me (for shooting a moving subject in low light) or I need the extra background blur one of the fixed focal length lenses will provide. The optical performance of Canon's L zoom lenses is outstanding - they get the job done best for me most of the time.
There are valid reasons for owning either or both lens types - You ultimately must decide what is best for you.
I hope this article has helped you compare Canon's zoom and fixed focal length lenses. Again, read my individual Canon lens reviews to find out which lens is best for you in the focal length you need.