Zoom lenses are now so common that we take them for granted. Even simple compact cameras have the option to zoom in or out of a scene. Cameras with interchangeable lenses typically come with a zoom lens covering a moderate range, such as 28mm to 100mm to focal lengths.
But as recently as 20 years ago the standard lens supplied with a 35mm camera had a fixed focal length of 50mm, which captured the scene being photographed pretty much as the eye saw it.
To have the option of taking wide-angle and telephoto pictures, you needed other lenses of fixed focal lengths. That meant extra expense and more weight to carry around. And there was sometimes a risk that when you saw something to photograph, by the time you had removed one lens and attached a different one to the camera, the bird could have flown or the sun disappeared behind a cloud.
Zoom lenses were around in the 1980s, but unless you had lots of disposable cash you were restricted to cheaper brands where image quality was inferior, particularly in the higher telephoto range.
Today optics have improved so much that zoom lenses match non-zoom lenses for image quality. That even holds true for modern lenses that cover a huge range, such as 28mm to 300mm.
Zoom lenses have always been a boon to people who use slide film. A print from a negative can be cropped to make the main interest fill more of the picture. Cropping slides is fairly easy, but if you hide too much of the original you end up with a tiny image on the screen or in the viewer.
With a zoom lens you can crop the image before you open the camera shutter. If you don't want a distracting lamppost in the picture, you simply zoom in a bit more.
Digital photography makes cropping simple, so in that sense zoom lenses are not as important as before.
But a big drawback of digital cameras with removable lenses is that the sensor, the bit which detects the incoming light, attracts dust. Even a tiny speck causes a blemish on every picture, and it won't move of its own accord because there are no moving parts in that area of the camera. Cleaning the sensor requires great care.
Each time you change your lens, dust is likely to enter the area in front of the sensor. So the less often you change lenses the better. A zoom with a good range can stay on your camera most of the time and cater for most of the pictures you want to take.
Zoom lenses can bring out your creative side and encourage you to experiment. If you're standing near a horse, you can use the wide-angle facility to photograph the whole animal, perhaps with the head emphasised in relation to the body.
Then you could zoom in a little to photograph just the head, then zoom in really close on a feature such as an eye or an ear. If you had to change lenses for each of these shots, the chances are you'd take just one and move on.
Rhodri Clark.: How they work:Any camera lens has a focal length, the distance from the film to the main focal point of the lens. Zoom lenses allow you to increase or decrease the focal length, by moving the principal focal point. A short focal length, such as 28mm, throws light from a wide area onto the film. This lets you photograph more of a landscape, and more of any object in a situation where you can't move further away. It also emphasises perspective and enables you to make a small object loom large in a wider scene.
A focal length of 50mm produces a picture on 35mm film where the perspective will look normal.
A higher focal length, such as 200mm, is known as telephoto. It gathers light from a smaller area. This is great for picking out features in a landscape and photographing animals without disturbing them. It flattens perspective, so you can make hills look steeper. It also reduces the area of the picture that's in focus, which is useful for shots - especially portraits - where you want a subject to stand out sharp from the background.
Digital sensors are currently smaller than the area of a 35mm image, so the effective focal lengths of lenses are increased by about 50% when attached to a digital camera. For instance, a 200mm lens would function as a 300mm lens on a digital camera.