The debate over prime lenses versus zoom lenses tends to get bogged down in the absolute qualities of each lens type, ignoring the reality of the situations in which you actually use the lenses. Objectively, in terms of optical quality and performance, primes are always better than zooms. However, in real life a zoom will frequently deliver better results. That’s why you will almost never see photo reporters using prime lenses, for example. I’d like to take a look at why that is.
What is a prime, anyway?
First of all, what is a “prime” lens and what is a “zoom” lens? A prime is a lens with a fixed focal length. You can’t “zoom in” or “zoom out” with a prime. What you see is what you get. A zoom does what it says: It lets you zoom in and out, getting closer and further away from your subject without having to walk towards or away from the subject.
Building a prime is much simpler for the lens manufacturer. You need fewer optical elements and almost none of them need to move, because the only adjustment is focusing. This makes it easier to make a lens that lets a lot of light in, and easier to control the light to get a sharper image. By the same token, a zoom lens needs more glass elements and they need to move relative to one another in complex ways to achieve the zoom effect and to focus. So you get less light and because it has been deflected so many times the image is less sharp.
What restricts zooms?
The restrictions of zooms are simple physics: If you have more glass elements and more of them need to move to zoom and focus accurately you’re going to get less light and a less sharp image than with a prime lens under the same conditions — or you’re going to have to build a very big, heavy and expensive zoom to compensate. That’s why the zooms with continuous f2.8 apertures that most professionals use are relatively big and heavy compared to primes, and very expensive as well.
When the difference is a difference
However, the key point here is “under the same conditions”: If, and only if, you are able to take the photograph from exactly the right position to fill the frame you’re going to get a higher quality image with a prime than the same frame-filling image taken with a zoom. In the real world this only happens under controlled conditions (in the studio) or when you have plenty of time (landscape or still life). And even then you may need to work quickly: In the studio you may want to re-frame without changing your setup, or you can be working with a model who moves fast, and the lighting of landscapes can also change before you have time to move to frame your shot perfectly.
When zooms win, also in quality
That brings us to the huge advantage that zooms have over primes: They make it much, much easier to make use of the full frame of the image sensor (or of the film on an old camera). Yes, a prime lens will always give you a better image if you have time to move around to fill the entire frame. But if you can’t then you’re going to be cropping the image later. When you’re only using a half or even a third or a quarter of the available image area with a prime and almost the whole frame with a zoom, then it’s pretty much a given that the zoom image is going to be better and sharper, and its resolution is always going to be better. And a zoom will let you get many more shots that you wouldn’t have been able to get at all with a prime, because you just couldn’t get into the right position in time.
In addition to this, lens manufacturers have made huge advances in recent years. The quality of many current zoom lenses is amazing, so good that it is very difficult to notice the difference between a zoom and a prime except on a huge gallery print. You may see the difference clearly in laboratory lens tests focusing on a tiny part of the edge or corner of the frame, but you won’t see it when looking at photographs under any normal circumstances.
The bottom line
So, yes, if you are after perfection and have the time to achieve it, then a prime will still get better results. When I’m taking portraits in sessions where I can take my time I still love to use my Fujinon 56mm f1.2 prime because if I can fill the frame with it I can get results that I can’t get with a zoom, even a very good one like Fuji’s 16-55mm f2.8. But if you don’t have ideal conditions where you can make use of the full available frame, then it’s very likely that you’re still going to get better pictures, even optically, with a good zoom.