Originally posted by ChrisPlatt Marc, only you mentioned perspective. And I don't use pixels.
The concept is of course the same with film as with digital - you *can* crop a 28mm to yield the same FOV as a 300mm lens would have produced, and the results will have *identical* perspective, but will of course have less resolution because it is such a heavy crop. So the difference resolution remains the answer to question you posed about why we bother to have different focal length: yes, you *could* shoot everything with a 28 (or whatever wide angle lens you prefer) and crop to yield whatever FOV you prefer, but you'd lose resolution as compared to using a different focal length in the first place.
Now, it *is* true you didn't mention perspective at first - although of course you did later in posting the link to the Wikipedia article on the subject. But the discussion is at this point vague and diffused enough that we might as well return to your original actual statement:
Quote: Zooms make you lazy. Rather than walk to compose your shot, one tends to zoom for framing, with little regard for the effect of focal length.
OK, then, what "effect of focal length" did you mean?
Note I totally agree with the idea that zooms often make people lazy, and in fact we may actually agree as to the reasons, but it's hard to say because you were kind of vague in your refer to "effect of focal length". Here's how *I* would put the issue:
Zooms can indeed encourage some bad habits. When encountering a scene one wishes to shoot, there are two very separate things one should really consider in composing it: what kind of perspective you want, and what kind of framing you want. These are *entirely* distinct things, as any artist could tell you: perspective is determined solely by where you position yourself, and framing is determined by the angle of view you choose to represent from that position. One could call these two things - perspective and framing - the fundamental aspects of composition.
What a zoom does is allow you to get the framing you want by changing angle of view from your starting position - which is to say, it allows you to choose any framing you want *without* encouraging you to first walk around to establish your perspective. So you end up not actually exercise creative control over one of the two fundamental aspects of composition.
On the other hand, if all you have a a single prime, you are *forced* into compromising one or the other. That is, to get a given framing, there is only one position from which you can achieve it, so you cannot have independent creative control over perspective. Conversely, to get a given perspective, there is only one angle of view your lens can produce, so you cannot have independent creative control over framing.
This is why I say that zooms - while they *can* encourage bad habits - are actually the better tools if you want to control both framing and perspective. The way to do this is to first walk around to choose your perspective, and *then* zoom to choose your framing. Of course, you may have already had the framing in mind before you started walking, but the point is, you don't actually select focal length until you select your position.
Of course, with a sufficiently large set of primes, one can do more or less the same. Especially if one considers the possibility of cropping, or the reality that a small change in position to fine tune framing might well not really mess up perspective too badly, one can generally get by with a surprisingly limited set of primes and still retain as much independent control of perspective and framing as one desires.
For me, in shooting landscape, experience with zooms - where I have full control over both perspective and framing - has taught me than I can get an awful lot done with just 28mm and 40mm (APS-C; 42mm and 60mm respectively for 35mm film in terms of reproducing the same framing for a given perspective). Others may find different focal lengths work best for them. Once you've identified the focal lengths that work best, you might indeed find you simply *prefer* working with primes (as you do, and I do tool). But zooms - if used "properly" to give independent control of perspective and framing - are a great teaching tool to help you find the focal lengths that seem to work best for you, and thus help guide your selection of primes.
But assuming the IQ, size/weight are within what you are willing to accept, you retain more control of perspective & framing with a zoom.