One of the main benefits of buying an SLR camera is that you can change lenses. Cameras usually come with a zoom that covers the focal range most people need, from wideangle to moderate telephoto. These are called ‘standard’ zooms or, when they’re sold with the camera, as ‘kit’ lenses.

What does the ‘focal length’ mean? You can tell whether a lens is a wideangle or a telephoto from its focal length. For example, ‘wideangle’ lenses have focal lengths of 35mm or shorter, while telephotos are generally 60mm or longer. The shorter the focal length, the wider its angle of view; the longer the focal length, the greater the magnification of distant objects.

‘Standard’ or ‘kit’ lenses generally cover the range 28-85mm, so with one lens you can shoot wideangle pictures, moderate telephoto shots and everything in between.

The ‘kit’ lenses supplied with most SLRs have a 3x zoom range which allows everything from wideangle shots (left) to short telephoto shots (right). For wider or longer shots you’ll need to invest in other lenses.

Digital SLRs do make this a little more complicated, though, because their sensors are smaller than 35mm film. This makes focal lengths appear ‘longer’ on these cameras, which is why camera and lens makers usually quote actual focal lengths for their lenses and ‘equivalent’ focal lengths too, so it’s easier to judge their angles of view. (There are two ‘full-frame’ digital cameras, the Canon EOS 1Ds MkIII and Nikon D3 where this doesn’t apply, but they’re expensive, professional models outside the scope of most photographers.)

The ‘kit lens’ with a Pentax K10D, for example, has a focal range of 18-55mm. But its ‘equivalent’ focal range is actually 28-85mm. This is all a bit of a nuisance, but as time goes by we will probably adjust to the new sizes and no longer have to mentally convert them to get an idea of a lens’s angles of view. In the meantime, there is an easy way to work this out. Just multiply the actual focal length by 1.6 to get its equivalent on a 35mm film camera.

1. 1.(From left to right): A ‘full-frame’ lens produces an image circle big enough to fill a 35mm film negative (the red rectangle).
   

2. 2.But the sensors in most digital cameras are smaller and use only the central section of the image produced by the lens.
   

3. 3.So makers produce ‘digital’ lenses with a smaller image circle to match the size of the smaller sensor. This makes a wider range of lens designs possible, but means that ‘digital’ lenses can’t be used on film or ‘full-frame’ cameras.
   

What’s a ‘digital’ lens? Some lenses will fit both 35mm film cameras and digital SLRs. These are generally lenses which have been available for some time and which were originally developed for film cameras. They will work perfectly well on digital cameras, but their focal ranges may not be ideal.

For example, the Tamron 28-200mm lens is ideal as a long-range zoom for a film camera because it goes from a 28mm wideangle to a 200mm medium-long telephoto. On a digital SLR, though, the equivalent focal range is 45-320mm. It’s even better for telephoto shots, then, but no good for wideangle pictures.

Instead, the ideal long-range zoom for digital SLR users would be the Tamron 18-250mm. This offers the equivalent of a 28-400mm lens, so that you can take wideangle shots and long-range telephoto pictures with a single lens.

However, this is a ‘digital-only’ lens. It can’t be used on a 35mm film camera or, for that matter, a full-frame digital SLR. It’s designed specifically for the smaller sensor size of a digital SLR and won’t cover a larger area. It’s not because the makers are being awkward, but because a lens which covered this focal range and covered a full 35mm frame would be impossible to make. 

Many more ‘digital only’ lenses are now appearing as digital SLRs are now more numerous and popular than film cameras. With this in mind, there are two things to remember when choosing lenses:

1. 1)Lenses designed for film cameras will also work on digital SLRs, but may not offer an ideal focal range, notably for wideangle shots.
   

2. 2)‘Digital’ lenses have ideal focal ranges for digital SLRs, but will not work on film cameras or full-frame digital SLRs.
   

What about lens aperture? This is the other measurement quoted by lens makers, so that the Pentax kit lens, for example, is an 18-55mm f3.5-5.6. Why two figures? Because for most zooms, the maximum aperture is smaller at the telephoto end of the zoom range. What those figures mean is that the lens’s maximum aperture is f3.5 at 18mm but falls to f5.6 at 55mm.

This isn’t ideal, but when taking pictures in program AE mode, you probably won’t notice. It’s only when you use Manual mode or Aperture-priority mode that it becomes a consideration.

Obviously, it would be better for lenses to have a constant maximum aperture, but this does make them much more expensive, and heavier too. The Pentax 16-50mm f2.8, for example, is much longer than the 18-55mm f3.5-5.6, and costs around £500.

Lenses with a large maximum aperture cost a lot more money, especially if they maintain the same maximum aperture throughout their zoom range.

What about lens fittings? Each camera maker uses their own lens fitting, which is incompatible with anyone else’s. Nikon lenses will only fit Nikon cameras, for example, while Canon lenses will only fit Canon’s. However, lenses from third-party manufacturers like Tamron can be bought in different mounts to fit different makes of camera - but you need to make sure you specify the right lens mount when you buy.

So what lenses do you need? If you find your camera’s ‘standard’ or ‘kit’ zoom often doesn’t go ‘wide’ enough for the shots you like to take, you should consider a wideangle zoom. On a digital SLR, the Tamron 11-18mm offers the equivalent of a 17-28mm zoom, which should be wide enough for almost any kind of photography.

If you find your SLR’s kit zoom doesn’t go ‘wide’ enough, you need a wideangle zoom. This 11-18mm zoom costs around £300. Wideangle zooms are optically complex and expensive to manufacture.

Or, if you like to take shots at longer range than your kit zoom permits, consider a 55-200mm zoom or perhaps a 70-300mm. These are equivalent to 85-320mm or 110-480mm respectively. Few photographers will need longer focal lengths than that.

Many people choose a telephoto zoom as their second lens, but while these are effective for sports and wildlife photography, you may find that you don’t use them as often as you expect.

The alternative to a selection of lenses is a single ‘superzoom’ lens which does the same job as two or more. The Tamron 18-250mm is a good example. Note, though, that ‘superzooms’ never go wider than ‘standard’ or ‘kit’ zooms - they simply offer an extended telephoto range. Superzooms are usually heavier than kit zooms, and may suffer from slighly more distortion, though this may not always be apparent in the photos. Superzooms are very flexible, but make sure you need the extra focal range and that you don’t mind the extra weight.

The Tamron 18-250mm ‘superzoom’ is barely larger or heavier than a standard ‘kit’ zoom. Some superzoom lenses, though, can prove too bulky and heavy and not as practical for everyday photography as you might imagine.

1. (From left to right) A standard ‘kit’ zoom will cover the majority of focal lengths you’ll need in everyday photography.

2. 2.A wideangle zoom is invaluable for confined spaces, sweeping landscapes and dramatic perspectives, but they’re expensive and not everyone needs them.
   

3. 3.For longer telephoto shots a ‘superzoom’ lens offers everything from wideangle to telephoto shots, but make sure you don’t get one that’s too heavy and inconvenient for everyday use.
   

4. 4.A telephoto zoom is an ideal second lens for sports and wildlife fans.
   

5. 5.Pro lenses come in extreme focal lengths or with super-fast maximum apertures, and with price tags to match.
   

What about image stabilisers? Image stabilisation is becoming more common in cameras and lenses. Tiny sensors detect any camera movement during the exposure and correct by moving elements within the lens, or the camera’s sensor, to keep the picture sharp.

If the camera has image stabilisation built in, you don’t need a lens with an image stabiliser. Many Pentax and Olympus cameras, for example, have inbuilt image stabilisation. Canon and Nikon cameras, though, don’t, which is why these makers offer ‘stabilised’ lenses instead. These cost more, but they are worth having because they cut camera shake with telephoto shots or pictures taken in poor light. You still need a tripod for long exposures, but image stabilisers help in ‘marginal’ conditions where a tripod is impractical.

This is the stabilisation mechanism inside the Tamron 28-300mm VC lens. The same principle is used in ‘stabilised’ lenses from other makers. Typically, a ‘stablised’ lens will let you shoot 2-4 shutter speeds slower than usual before camera shake starts to appear.

What about lens quality? You read a lot about this, and whether one brand is better than another, and it’s true that differences do exist, but they’re often quite small. It’s more important to be a good photographer than to buy the best lenses. More expensive ‘pro’ lenses don’t just give you slightly better picture quality. They are made to withstand years of hard use and often come with wider maximum apertures than ‘amateur’ lenses. For example, a pro telephoto lens might have a maximum aperture of f2.8 or f4, where a cheaper ‘amateur’ zoom lens has a maximum of f5.6 or f6.3. The higher maximum aperture makes all the difference for pros shooting in low light - just think of a football match at 3pm on a January afternoon, or an indoor athletics meeting held under floodlights.

What’s the difference between ‘pro’ lenses and ‘amateur’ lenses? Pro lenses are designed for not just for optical quality but also for hard use, extreme conditions or both, and this is reflected in the price. This Canon 800mm f5.6 ultra-telephoto cost a staggering £9,000.

Still not sure? Here are two thoughts to help you when you’re considering investing in a new lens:

1. 1)Only buy a lens when you know you definitely need it, not when you think you might at some point in the future. In other words, don’t buy lenses ‘just in case’.
   

2. 2)Spend according to the use you’ll get from the lens. In other words, if you’re only going to use that super-long telephoto a few times a year, don’t spend a fortune. But if you’re going to use it every week, it might be worth paying a little extra.
   

Copyright © Rod Lawton 2008