Zoom lenses are undeniably great when it comes to convenience and versatility, delivering a wide range of focal lengths at the flick of a wrist. However, they demand a compromise in terms of outright image quality. With complex arrangements of large groups of lens elements moving back and forth to enable zoom, the optical purity suffers.
Sharpness is often the first casualty, and barrel and pin-cushion distortions often appear at the wide-angle and telephoto ends of the zoom range respectively (learn How to find your lens' sweet spot).
You can also expect an increase in chromatic aberration (colour fringing around high-contrast edges in a scene) and vignetting (see our photography cheat sheet explaining chromatic aberration). The latter effect is most commonly seen when you're using large apertures at the wide-angle end. Zoom lenses are also often more prone to ghosting and flare.
Switch to a high-quality prime lens, and distortion and vignetting should be much less noticeable. Sharpness should also be excellent, so you can really make the most of the high-resolution sensors fitted to current digital cameras.
Another big bonus of using prime lenses is that they're usually 'faster'. This means they have a larger maximum aperture, which enables quicker shutter speeds. For example, a typical 18-55mm zoom lens has a maximum aperture of roughly f/4 at the wide-angle end, shrinking to a mere f/5.6 at about 50mm. Switch to a 50mm f/1.4 prime lens and the largest available aperture is four stops faster.
In low light you'd be limited to a shutter speed of, say, 1/15 sec with a typical zoom (unless you increase your ISO setting). However, an f/1.4 lens will enable a much faster shutter speed of 1/250 sec. An f/1.8 lens is 3.3 stops faster than an f/5.6 lens, and even an f/2.8 model is two stops faster (for more on shooting in low light, see our guide to the 12 common errors of night photography - and how to fix them).
So-called 'faster' lenses aren't just good for avoiding camera-shake and freezing the action in dull lighting conditions. Another big advantage is that you can get a much tighter depth of field, enabling you to isolate the main point of interest in a shot by blurring the background (learn more about how to master depth of field). It's a favourite trick in portraiture, especially when the background is cluttered and would otherwise be a distraction (learn how to make the ultimate bokeh portrait).
It can be tricky to use large apertures in bright, sunny conditions, but you can get round the problem by fitting a neutral density (ND) filter. Variable NDs are a great option when you're shooting video and want the cinematic, shallow depth of field that's all the rage (find out How to set up your DSLR for video recording).
An important factor to consider when you're buying a prime lens is which focal length to go for. Back in the days of 35mm film, a 50mm prime was considered a 'standard' lens. That's because it gives pretty much the same perspective as viewing a scene with the human eye, without the magnification of a telephoto lens or the shrinkage a wide-angle lens uses to squeeze more into the frame.
With all that in mind, to help you get more from your fixed-focal length optic we've offered up below our 9 points you should know about using prime lenses.
9 things you should know about using prime lenses
1 Doors to manual
Many older prime lenses won't autofocus on some camera bodies - the Nikon D3100 and D5100 are examples of this - which can be rather limiting for general shooting (for more, see Manual Focus: what you need to know to get sharp images).
2 High frequency
Nikon's ring-type ultrasonic autofocus system is usually very fast, but in the brand's 50mm f/1.4G and f/1.8G it's a bit slower than you might expect. However, it's still fast enough for most types of shooting.
3 Bigger is better
The 'fastest' lenses have apertures of f/1.4 or f/1.8 and enable higher shutter speeds and reduced depth of field. This makes them more useful than f/2.8 lenses.
4 Play with light
Invest in a neutral density filter if you want to shoot with large apertures - it will reduce depth of field in sunny conditions.
5 To and fro
Being able to tweak the focus setting with full-time manual override is great. You can fine-tune autofocus settings without having to switch back and forth between AF and MF modes.
6 Close quarters
A macro facility adds versatility, but you'll have to be very close to the object you're shooting with a 50mm lens, and a 35mm is almost unusable.
7 Open wide
When shooting at the maximum aperture with fast f/1.4 lenses, outright sharpness can be a bit lacking.
8 Sitting pretty
A 50mm lens on an APS-C format body is a great combination for portraiture. A maximum aperture of f/1.4 or f/1.8 enables you to blur the background much more effectively than you'd be able to with a budget 18-55mm zoom lens.
9 Investment banking
It's a good idea to buy a professional optic, such as an FX-compatible lens, even if you currently use an APS-C format camera. It future-proofs you in case you ever decide to trade up to a full-frame camera.