In this post we answer the 7 key questions you should ask yourself before buying new lenses. We also include our new photography cheat sheet, which breaks down some of the key features to look for in new lenses.
Lenses may look like simple tubes with glass at each end, but a lens can house up to 20 or so individual lenses in a dozen or more groups.
Other parts include the aperture blades and an autofocus motor. Then there are all the electronics needed for communicating settings with your camera. Clearly, lenses are anything but simple, but fortunately you don’t need a PhD in optical engineering to understand how to use them.
The lens you choose to fit on your SLR will determine how your end results will look. The main consideration is focal length. Focal length is the distance between the optical centre of a lens and the sensor, but it’s easier to think in terms of how this affects subject size when compared with a standard lens (see below).
For a camera with a full-frame sensor, a standard lens is 50mm. Lenses with focal lengths shorter than 50mm are known as wide-angle lenses, while lenses with focal lengths longer than 50mm are referred to as telephoto lenses.
Which type you need to use will depend on the subject you’re shooting, and the distance you’re shooting from, but most focal lengths offer creative potential with all subjects.
7 questions to ask yourself about buying new lenses
01 What focal length should I choose?
Think about the type of photography you do and consider lenses with focal length ranges that will enable you to cover every eventuality. Broaden your lens collection so it covers the complete range for your chosen subject before splashing out on ‘faster’ lenses.
Upgrading your kit lens to a better quality lens with a similar range is also worth considering, however, as this will ensure better results at the most commonly used focal lengths (see page 90 for a full group test of standard lenses).
02 What lens ‘speed’ should I choose?
A lens with a maximum aperture in the range of f/1.2 to f/4 is often referred to as a ‘fast’ lens. Lenses with these wider maximum apertures let in lots of light, allowing you to set fast shutter speeds. They’re therefore useful for action photography and for shooting in low light.
Fast lenses are usually of a high quality, but they’re often bulky and expensive, so they should really only be bought (or hired) for specific subjects or occasions.
Slower lenses, meanwhile, tend to be much cheaper and lighter, although you may need to boost the ISO setting (potentially reducing the image quality because of an increase in image noise) to get the same fast shutter speeds.
As an example, if a lens with a maximum aperture of f/2.8 enables you to set a shutter speed of 1/125 sec at ISO100 for a given exposure, a lens with a maximum aperture of f/5.6 will only enable you to set the same shutter speed if you increase the ISO to 400.
Wider apertures also create smaller depths of field, which is especially useful in portraiture.
03 What is the optical quality like?
It’s only by testing a lens you’ve hired, by reading reviews and by speaking to photographers already using the lens that you’ll know if a lens’s quality will meet your expectations.
Sharpness is key, not just in the middle of the frame, but also at the edges, but things like vignetting and other optical aberrations are important too.
04 What is the minimum focusing distance?
How close can you get to your subject before the lens reaches its focusing limit? This is an important consideration when choosing a telephoto lens.
It’s all very well getting a lens that offers a huge magnification, but if you can’t get close enough to make the most of it (shooting portraits, for example), then you’ll have to add extension tubes to get closer.
05 Does the filter ring rotate?
Cheaper lenses often have a front element that rotates as the lens focuses. This causes problems when you’ve got a filter attached – particularly a polariser, the effect of which changes as it turns. The only solution here is to focus first, then turn the autofocus off by switching to manual focus, before rotating the polariser.
06 How much should I spend, and will it make a difference?
Considering that many kit lenses supplied with DSLR bodies cost little more than £100, what kind of benefits can you expect by spending £1,000 or more on new lenses?
For starters, buying a luxury lens will give you a fast constant aperture throughout the zoom range, whereas entry-level lenses tend to be much slower – f/3.5 at their widest angle and f/5.6 or slower at the telephoto end.
This makes them less useful in low light photography, and all but useless if you want to achieve a shallow depth of field. High-end lenses should also be more resistant to chromatic aberration, lens distortion and flare. But remember, there’s no such thing as the perfect lens.
07 How does it handle?
How a lens feels shouldn’t be overlooked. How heavy is it, for instance? Is the focusing ring smooth or jerky? What’s the zoom action like? Cheap zoom lenses can be a bit wobbly, with too much sideways movement in the front element of the lens when you zoom in.