Lenses with focal lengths above 50mm are commonly known as telephoto models. These fit less in, and are therefore ideal for getting closer to distant subjects or picking out detail; they also give a more flattering effect when taking photos of people. In contrast to wide-angle, lenses with longer focal lengths have an inherently smaller depth-of-field, which means it’s easier to get a blurred background effect – again ideal for portrait, wildlife and sports photography.

Good focal lengths for portraits are typically between 85mm and 135mm – these are often known as short telephotos. Appropriate focal lengths for sports or wildlife are generally much longer – at least 200mm, and ideally 300mm or more. Professional sports and wildlife photographers often use 600mm lenses, or even longer still. These are known as super-telephotos.

Lenses with smaller f-numbers gather more light thanks to their bigger apertures, which makes them better in dim conditions and for minimising the depth-of-field. Bigger apertures however mean a larger, heavier and more expensive lens.

As focal lengths increase, the risk of camera-shake becomes greater. Sony's Alpha DSLRs feature built-in anti-shake systems which allow you to handhold shots at slower than normal shutter speeds, but remember these won’t stop a moving subject from blurring, which brings many enthusiasts back to large aperture models.

As always, bear in mind any reduction in field due to your DSLRs sensor when choosing a lens. Sony’s APS-C models like the Alpha A200, A300, A350 and A700 have a reduction factor of 1.5x, which effectively multiply focal lengths by 1.5 times. So if you’re after a portrait lens with an equivalent focal length of 75 to 128mm, you’ll need a model with an actual focal length of around 50 to 85mm. Full-frame bodies like the A900 have no field-reduction factor, so an 85mm lens will deliver 85mm coverage.

Complete Sony lens range / DSLR Tips lens buyers guide