Back to Basics: What is the difference between zoom lenses and prime lenses?

50 mm prime (left) and 35-70 mm zoom on lens (right).

Contemporary camera lenses have evolved into two distinct lens families, zoom lenses and prime lenses. Zoom lenses encompasses all camera lenses that allow users to adjust the focal length on-the-fly, while prime lenses possess a fixed focal length, one that cannot be changed by the operator without the use of a hammer… although I wouldn't suggest that!

A common misconception is that the zoom in zoom lens specifically refers to lenses with long focal lengths: 200, 300, 400 mm and so on… but the truth is that all camera lenses that feature continuously variable focal lengths are zoom lenses. A 10‑18 mm, for example, is every bit as much a zoom as a 100-400 mm might be.

Zoom lenses owe their popularity in present-day photography to one key benefit, the ability to change the composition of a photograph from a fixed location. By zooming in an out, the appearance of the subject in an image can be altered (in some cases to the point of being unrecognisable) without the photographer needing to move their feet. A similar accomplishment using prime lenses would, at the very least, require swopping one lens for another, but even then, a bag full of primes would simply not match the endless variety of focal lengths offered by its zooming counterparts.

Prime lenses are sold in discrete focal lengths: 24, 35, 50 mm (to list but a few), which means that if you are in need of a 27.5 mm focal length, for example, you are going to have to find yourself a gifted glassblower. By comparison, a 16-35 mm zoom lens would have no trouble with a 27.5 mm constraint, assuming, of course, you have the prehensile dexterity to twist the zoom ring with robot-like precision.

To be honest, my illustration of the primary prime-lens pitfall is an exaggeration and not at all representative of real-world photography; the principle, however, is sound. One zoom lens has the potential to outperform a whole collection of prime lenses, at least in terms of focal-length flexibility, and it is also likely that the solitary zoom will be a lighter, less expensive alternative to a bag full of primes.

With so many advantages attributable to zoom-lens ownership, the question of why manufacturers continue to produce prime lenses is a valid one. The answer is that despite their shortcomings, primes still possess a host of attractive benefits.

A cross section of a zoom lens showing internal optical elements.

Chief among these benefits is image quality. When engineers set about designing a zoom lens, optical compromises have to be made to ensure that the resulting lens is acceptably sharp throughout its zoom range, and that any distortion is also kept within tolerable levels. When designing a prime lens, on the other hand, there is no need to compromise. Engineers can optimise the design to achieve the best possible image quality at the designated focal length.

The second major advantage of prime lenses is found with their maximum aperture. While there are no physical properties limiting the maximum size of a zoom lens’s aperture, both the mechanism responsible for the zooming action as well as the sheer number of optical elements present within a zoom places practical limitations on the ultimate size of the diaphragm. Zoom lenses are rarely found with maximum apertures greater than f/2.8, whereas prime lenses tend to only exist at apertures larger than f/2.8. Maximum aperture values of f/1.8, f/1.4 and f/1.2 are all common-place in the prime-lens family and perhaps no lens illustrates just how large the aperture opening of a prime can be better than the extinct Canon 50 mm f/0.95, last marketed in August 1961. Sometimes referred to as the “dream lens”, the 50 mm f/0.95 was equipped with a truly gargantuan orifice.

Canon 7 with the Canon 50mm f/0.95 lens. Image Attribution: Wikimedia Commons.

However, despite the unwieldy appearance of the dream lens, a further benefit often associated with more pedestrian incarnations of the prime lens is a compact form factor. With fewer optical elements comprising the complete lens assembly, primes can be considerably smaller and lighter than their zoom-lens equivalents. Naturally, the slender lines come at the cost of focal-length freedom, but there are many circumstances under which the trade-off is worthwhile. Smaller, inconspicuous lenses are undoubtedly useful, with street photographers being particularly fond of a surreptitious optic.

But, before this article begins sounding like an advert for prime lenses, I must point out that not all prime lenses are created equal. The advantages that I have outlined here are, without question, a significant generalisation. I have personally bought and sold prime lenses that have failed in every way to live up to the prime-lens promise and, coincidentally, the sharpest lens in my bag today is a 70-200 mm zoom lens, a fact that speaks volumes about said generalisation.

The Canon EF 70-200mm f/4L USM, the sharpest lens in my bag.

The reality is that zoom lenses and prime lenses both have a place in the modern camera bag. There is little that a prime lens can do to surpass the versatility of a zoom lens; however, there is also no better way to experiment with large apertures than to own a fast prime.[1] Since almost all photography today starts out with a zoom lens, I encourage all those taking their formative steps into the art to do a little research and to buy at least one good prime. Owning a prime is a piece of photography experience that every photographer ought to have. 

Footnote

[1] Lenses that feature large maximum apertures are referred to as ‘fast lenses’. The name is derived from the fact that large apertures allow a lot of light to enter the camera and consequently permit the use of fast shutter speeds.

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