This question torments a great deal of lens buyers – should I purchase a zoom lens or a fixed focal lengthe (prime) lens? When it comes to versatility and convenience, zoom lenses deliver a wide range of focal lengths at the flick of your wrist without having to constantly change lenses. Thus, quickly enabling you to frame and capture a moment with the appropriate focal length. This is a advantage for zoom lenses but also their biggest weakness as there is a compromise in image quality. Complex arrangements of large groups of elements inside the lens are moving back and forth to enable the zoom and these reduce the optical quality of your images.
Despite their versatility zoom lenses can cause a several issues with your photography. The sharpness of your images is the first victim, barrel and pin cushion distortions can often appear at the wide-angle and telephoto ends of your lens zoom range. Zoom lenses can also cause vignetting, a reduction of an image’s brightness or saturation. You can also expect an increase in chromatic aberration (known as colour fringing around high-contrast edges in a scene). This effect is most commonly seen when you’re using large apertures at the wide-angle end. Zoom lenses are also more susceptible to ghosting and flare.
If you choose a high-quality prime lens, distortion and vignetting will be much less noticeable due to fewer moving parts and prime lenses are optimised to a specific focal length or purpose. This means optical performance is generally better and lenses can be made with larger apertures. The sharpness of your images will be unparalleled, so you can really make the most of the high-resolution sensors fitted to current digital SLR cameras.
Another advantage of using prime lenses is that they’re generally ‘faster’, which means they have a larger maximum aperture enabling faster 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. If you switch to a 50mm f/1.4 prime lens the largest available aperture is four stops faster. In low light you’d be limited to a shutter speed of 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.
One more advantage is that you can get a much more concentrated depth of field, enabling you to isolate the main point of interest in a shot by softening the background. It’s common practice in portraiture, especially when the background is busy and would otherwise be a distraction.
An important factor to consider when you’re buying a prime lens is which focal length to choose. Back in the old days of 35mm film, a 50mm prime was considered a ‘standard’ lens. That’s because it gives 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. I would recommend the following lenses: For landscapes – 18mm or 21mm. Full body fashion/editorial shots – 35mm. For portraiture half length – 50mm is great but for tightly framed head and shoulders an 80mm, 85mm, 100mm or 110mm is best. As a professional portrait photographer I keep an 80mm F2.8 and 110mm F2.8 in my bag. These are the best two portrait prime options. The 80mm great for editorial portrait photography, capturing the person and some of the setting, and the 110mm for closely framed head-shots.