The Case For and Against Zoom Lenses
Some photographers have strong positions, on one side or the other, regarding the use of zoom lenses. Let's see if we can get to the bottom of this. Or at least I can tell you where I stand. And why.
There once was a time when zoom lenses didn't even exist. All lenses were fixed focal length lenses so photographers had to find other issues to argue about. When variable focal length (zoom) lenses did come on the scene, there still wasn't much of an argument. Although novel in concept, zoom lenses were extremely limited in quality. Changing focal length requires moving major lens elements closer and further from the point of focus, and optical designs were really only optimized over a limited range. As you moved further away from that "sweet spot," a variety of optical aberrations started to manifest. It didn't really matter if it were softening of focus or vignetting in the corners of the frame or something else, nobody could argue that zoom lens quality couldn't measure up to fixed focal length (prime) lens quality. Zoom lenses were an invention of convenience, not of improved quality.
As the years went by, the computer revolution began to exert an influence on just about everything, including photography. That revolution of course has given us digital cameras, but it has also had a significant impact on lens design. Rather than relying on the skill of expert optical designers together with trial and error to iteratively improve lens designs, it became possible to use computer software to mathematically determine the optimal lens design for any given need. The manufacturing process is also now computer controlled, creating levels of precision and repeatability not possible earlier.
So now we live in a world where some of the sharpest lenses available are zoom lenses. Prime lenses haven't gotten any worse of course and some of them still rank up there too, but now the playing field have become more leveled. Generally speaking, newer lens designs are sharper than older designs simply because they are newer, regardless of whether they are prime or zoom designs. So we have reached a point where both prime and zoom lenses receive praise on DxOMark and other lens testing sites. The highest quality lenses though remain prime lenses, but the difference between them and their zoom equivalents has definitely narrowed.
There's still the convenience factor of zoom lenses of course. Prime lens advocates still have other valid arguments on their side too. Having all that extra glass and scaffolding to move it around precisely leads to zoom lenses weighing more than equivalent primes. That can matter in the field if you have to carry all that extra weight around with you. These same factors can also lead to good quality zoom lenses costing more than equivalent primes. That can matter in your wallet if you have to pay for that added convenience.
Prime lens aficionados often argue that zoom lenses lead to bad habits. It can be too easy, they say, to stand in one place and let the lens do the framing work for you. The only good way to explore the photographic potential of a subject is to move your feet and examine it from every angle. A lazy photographer though can use the zoom capability of a lens to avoid all that hard work, zooming in or out until the subject fills the frame and then snapping away at the shutter release. And all this from wherever you may be standing, all without ever exploring other vantage points.
It should be noted that zooming your lens while standing in one place does not produce the same effect as you get from changing your shooting position by moving closer or further away from your subject. While both allow you to better compose your subject by filling the frame, they do not have the same effect when it comes to everything else in the frame. Zooming increases or decreases the size of everything by equal amounts. Objects in the foreground and background get bigger or smaller together, by equal degrees. Getting close or further away from your subject will also change its apparent size, but other objects in the frame may react differently. Changing subject distance results in things getting bigger or smaller proportionally to their distance from the camera lens. A foreground flower will change radically in size with even a modest change in shooting distance while a mountain in the background will be barely affected.
So if moving your feet and zooming your lens don't even do the same thing, is it reasonable to compare which is better? In my opinion, no. And yet even experienced photographers can be found that promote the use of prime lenses to prevent developing bad habits from zooming. To me, both are indispensable tools and comparing one to the other only serves to confuse what each one does. It would seem preferable therefore focus on teaching both techniques and using each when appropriate. Zooming before moving your feet can indeed rob you of the opportunity to fully explore your subject, but neither technique can really substitute for the other.
Here's how I approach things. Shooting distance determines relative perspective. By getting closer or by backing up, you can make your subject bigger or smaller relative to other objects visible in the frame. It allows you to control the visual impact of your subject. Do you want it to look big and imposing, or small and weak? Do you want to see it up close in detail, or from further back but in context? The choice is yours. Only once this has been settled on is it appropriate to reach for the zoom ring. Zooming after settling on relative perspective lets you increase everything together with that perspective until the frame as a whole is comfortably filled.
Sometimes compromises do need to be made though. Shooting in the field means I have to contend with the terrain and other limitations that exists in the field. If the ideal shooting location would mean I have to stand over the edge of a cliff, I may not be able to get the optimal relative perspective and have to compensate with an added degree of zooming. The same reasoning holds when confronted by a sign telling me to stay on the trail to avoid damage to fragile mountain soils. You get the idea. To a degree, you can compensate for changes in shooting distance through the use of lens zooming and vice versa. But to the extent possible, I try to concentrate on optimizing one before really thinking at all about the other. First, move your feet, then move the zoom ring.
So I guess that means I fall on the zoom lens side of the prime versus zoom debate, but only with caveats. Zoom lenses force the use of shooting distance to determine framing but they also limit what you can easily do once that variable has been optimized to the extent possible. Sure, you can switch to a different prime lens once you've walked forward and back to arrive at your selected shooting distance, but there are limits to doing so since you would be unlikely to have as varied of a selection of prime lenses with you to change focal length as could be achieved with even a modest range of zoom lenses. And even if you did, the entire argument about prime lenses forcing you to properly select your shooting distance would lose its force. You could just as easily stand in one spot and go through your arsenal of primes as you could by just standing there while rotating the zoom ring.
The true answer comes from better understanding what each method does. Zooming and walking are not interchangeable.