Lenses are specified by their focal length (and a zoom lens will cover a range of focal lengths). The longer the focal length, the higher the magnification of the object you are photographing. So to photograph distant objects, be they swimmers or anything else, what you need is a lens with a long focal length.

So what constitutes a long focal length? Well there could be a long discussion on this, including considerations such as sensor size and field of view, but that might be beyond the scope of this explanation. For really effective photography of distant objects, I would suggest simply looking at lenses of 200mm focal length and up. If possible, go to a camera shop and try some out to see what you think might suit your needs.

Other considerations that you should have include:

• Maximum aperture - a wide maximum aperture (such as f/2.8) will allow more light into the lens, allowing for faster shutter speeds, and consequently allowing you to freeze action and minimise camera shake.

• Image stabilisation - this feature is another way to counteract camera shake, allowing you to use slightly slower shutter speeds (which can be useful when ambient light levels are lower).

One thing you don't need to be concerned about is flash. On-camera flash has no effect at longer distances. (In fact it has a negative effect, as it fools your camera into thinking that the scene is being artificially lit when it is not.) You will achieve better exposure without flash.

One last point, you say you're not sure what to ask for in a shop. That's exactly what shop assistants are there for!