Choosing a lens

Most of us start off with a cheap everyday lens when we get into digital photography. The kit lenses that come bundled with camera bodies are versatile workhorses, with decent zoom ranges that will be useful in most photographic situations. However, there will come a time when you want to add to your creative equipment, and expanding your collection of lenses is a great way to do this. The lens you opt to put on your SLR is one of the most important creative decisions you can make. Other than the sensor, it will have the greatest influence over the quality of your images. Here’s a brief introduction to some of the key things you need to consider when choosing a new lens.




Crop factor
The size of your SLR’s sensor affects the angle of view of your lens – the smaller the sensor, the longer the ‘effective focal length’. A ‘full-frame’ sensor, as used in the Canon EOS 5D mark II or Nikon D700, has the same physical dimensions as a frame of 35mm film, but smaller ‘APS-C’ sensors, as used in the majority of entry-level and mid-range SLRs, capture a smaller proportion of the image projected by the lens. This has the effect of increasing the effective focal length by a factor of 1.5x or 1.6x (depending on the manufacturer). On a camera with an APS-C sensor, a 50mm lens effectively provides an angle of view equivalent to a 75mm or 80mm lens on a full-frame or 35mm film camera. This is great news if you want to shoot wildlife or sports, because a 400mm lens effectively becomes a 600mm or 640mm lens. However, it means that for serious wide-angle work, you need to use an ultra-wide lens – which is why lenses such as the Sigma 10-20mm are popular with landscape photographers who use cameras with APS-C sensors.
Lens speed
We’re not talking focusing speed here, but rather the light-gathering capability of the lens. ‘Fast’ lenses have wide maximum apertures, such as f/1.4 or f/2.8, and so on. They let in more light at this aperture than lenses with maximum apertures of, say, f/4 or f/5.6, which enables you to achieve faster shutter speeds without increasing the ISO. The downside is that they are heavier and more expensive. With ‘slower’ lenses you may need to increase the camera’s ISO in order to get sharp shots in low light. The speed of a lens is also relative to its focal length – a 500mm f/4 lens is considered fast, whereas a 100mm f/4 lens is considered slow. The wide maximum apertures of fast lenses also enable you to achieve a shallower depth of field in your pictures – great for portraits, sport and wildlife.
Floating or fixed aperture?
Another factor that determines the cost of a zoom lens is whether it maintains the same maximum aperture (f/2.8, for example) throughout the zoom range (more expensive) or if the aperture gets smaller (f/4-5.6, for example) as you zoom from wide to long focal lengths (cheaper). The downside of so-called ‘floating’ apertures is that in order to maintain the same exposure, the shutter speed needs to be decreased as the aperture gets smaller.
Five lenses worth saving for…

Telephoto zoom
A telephoto lens is an optic with a focal length of over 70mm. ‘Tele’ means ‘far off’ in Greek, so telephoto lenses make far-off subjects seem closer than they really are. They also compress perspective.

Wide-angle
Anything within the 10mm to 24mm range is considered to be a wide-angle lens (on APS-C SLRs), which makes wide angles a natural choice for landscapes and shots taken indoors.

Macro
True macro lenses will enable 1:1 magnification in close-up photography, so subjects appear life-size on the sensor. This means that you can fill the frame with objects that are an inch or two wide.

Prime
Prime lenses have fixed focal lengths (which means they can’t be zoomed). They tend to have fast maximum apertures and are generally of superior optical quality compared with zoom models.

Superzoom
Superzooms offer an enormous range of focal lengths (such as 18-200mm) in a single lens, which makes them ideal for travel photography. The downsides include narrower apertures and inferior quality.
 

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