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Lens Fundamentals: Prime Or Zoom?

The meaningful differences between fixed focal length lenses and zooms

A debate has raged for years about which is better, prime or zoom lenses. For serious professionals of a certain age, prime lenses (which have a single fixed focal length, such as a 35mm, 50mm or 85mm) are seen as head and shoulders above zooms (which have adjustable focal lengths in a range, such as 24-70mm or 70-200mm, two very popular zoom ranges). This is because photographers who were raised in the darkroom were working in an era where the engineering of zoom lenses wasn’t nearly as advanced as it is today. Consequently, zooms inevitably were less sharp and produced more vignetting, chromatic aberration and other issues that earned them the reputation of being inferior in quality compared to primes.

But that was then. These days, and for several years now, those brilliant engineers at Canon, Nikon, Sony et al. have figured out how to produce zoom lenses of incredible quality. So while the most distortion-free and aberration-free lenses in any manufacturer’s lineup may still largely be primes, they’re no longer exclusively primes. Zoom lenses have come a long, long way.

Look at the work of lens testing firm DXOMARK, which evaluates lenses on a number of technical criteria to determine an overall quality score. Take its Sony lens rankings, for instance. (These can be found online at www.dxomark.com/Lenses for any manufacturer.) There are two zoom lenses in the brand’s top 10, a level of quality that would likely have been unheard of just a decade ago. And as engineering and manufacturing capabilities improve, that list of top-quality zooms is likely to grow—especially because zooms are so popular.

One downside of any manufacturer’s best zoom lenses is that they’re quite expensive. And one corresponding rule of thumb is that cheap zoom lenses are rarely of the highest quality. That’s not true for inexpensive prime lenses, which in some cases are very good. This is because it’s easier and less expensive to produce a smaller, faster (with a wider maximum aperture) and sharper prime lens. A sharp 50mm prime might only cost $250, while a similarly sharp telephoto zoom could easily sell for 10 times that. It’s for this reason that some folks prefer primes to zooms: they can outfit their kits for less.

But the counterargument is equally simple: A single 24-70mm zoom lens, for instance, accomplishes the work of at least four prime lenses (a 24, 35, 50 and 70mm prime). So when comparing the cost of, say, a $2,000 24-70mm zoom lens, one should really compare it to the cost of buying all four of the aforementioned primes. In that light, the expensive zoom doesn’t seem so cost prohibitive.

Similarly, traveling photographers might find traveling with two zooms (a 24-70mm and a 70-200mm) much preferable to strapping on a shoulder bag containing a 24, 28, 35, 50, 70, 85, 100, 135, 150 and 200mm lens. Not only would that present a preposterous hit to the pocketbook, it would be preposterous to travel regularly with 10 primes on your shoulder. Even though zooms are typically larger than primes, when it comes to traveling light without sacrificing focal range, the advantage goes distinctly to the zooms.

What about working with these lenses. Are there fundamental differences to the look of a shot made on a 70-200mm zoom and, say, one made with an 85mm prime? Again, the answer is “sort of.”

Because primes are easier to make, they’re easier to make great. And one way manufacturers make them great is to make them really, really fast—with maximum apertures of, say, ƒ/1.2. A lens that fast can shoot with faster shutter speeds, in lower light and with shallower depth of field than even a great, fast ƒ/2.8 zoom. That’s nearly two stops faster than an already fast ƒ/2.8 zoom. If you’re comparing it to a slower zoom with a maximum aperture of ƒ/4, or even a variable maximum aperture of, say, ƒ/5.6-6.3, you might be giving up three or even four stops of speed—and that’s a big deal when you’re working in available light at the edges of the day or when you’re trying to throw the background out of focus for a portrait. It’s for this reason, frankly, that prime lenses are still as useful as ever. For those who need the speed to work in low light or to create incredibly shallow depth of field, there’s just nothing that comes close to a fast prime.

There’s one more thing about the prime versus zoom debate that photographers should really consider, and that’s how they’ll think about composition when using a given lens. I find with a zoom lens I tend to stand still and adjust the focal length to change the framing as needed. This flexibility is actually one of the great benefits of zooms. But when I’ve got a prime lens tied on, I’m much more likely to move around more, to “zoom with my feet,” and explore a scene more actively in order to change compositions and fill the frame as needed. And I find this change in the way I think about shooting to be quite gratifying and a fundamentally different way to work. It really helps me to have the feeling of exploring with my camera. It’s not necessarily better, but it’s definitely different.

Perhaps it’s as simple as the lack of one more variable that makes the prime lens process different. With a zoom lens, you’ve got to make a fundamental decision every time: am I shooting at 24mm, 35mm, 50mm or somewhere in between? That decision is removed when you put on your 35mm lens because it’s only that focal length. And so this “limitation” may actually be freeing, allowing the photographer to focus more on composition and the contents of the frame.

Ultimately, there’s no correct answer in this everlasting debate, and there are certainly places for both primes and zooms in most photographers’ kits. But there’s never been a better time to take advantage of the impressive image quality both lens categories are capable of. The even better news is that lenses are only going to keep getting better.

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