What is focal length? It's the lens that determines how much of the scene you see in your viewfinder, and ensures that it is recorded in sharp focus.
Different lenses produce differing angles of view. Some lenses show a wide area of a view (wideangle), others a narrower crop (telephoto). The angle a lens can cover depends on its focal length, which is measured in millimetres.
The precise focal length required for a specific angle of view varies depending on the size of the sensor used in the camera.
You will often see cameras and lenses list, along with the actual focal length, the equivalent focal length on a 35mm camera, since this is a known fixed quantity that many photographers are familiar with. Most modern lenses are zooms, so you can change the focal length by turning a ring around the lens.
The zoom range available varies with each lens but the kit lens that comes with most DSLRs can zoom from moderate wideangle (around 18mm) to short telephoto (about 55mm), though longer-ranging zooms are available. You can also buy fixed lenses of a single focal length.
The mid-point of most kit zooms produces a point of view roughly equal to what the eye sees, so subjects appear to be at about the same size and distance when looking through the camera as they do to the eye. On a typical enthusiast DSLR this equates to a focal length of around 35mm, or 50mm on a full-frame DSLR.
The wideangle end of your zoom (towards 18mm) makes subjects look smaller and further away. It’s ideal when you need to get more into your shot.
Wideangles emphasise subjects closer to the lens by making them appear disproportionately larger than objects further away, thus exaggerating the sense of scale and distance. They can accentuate or distort lines and curves within a scene, and also produce greater apparent depth of field.
If the widest end of your lens still isn’t wide enough you can buy lenses that go wider, but the more wideangle the lens, the more pronounced these optical characteristics will be.
As you zoom in you exclude more and more of a scene, as distant elements become larger in the frame and appear closer.
If your zoom isn’t long enough you can achieve the same effect by cropping your photo on the PC later, though of course doing it this way will reduce the image quality.
The telephoto end of your zoom will have the opposite effect to the wideangle end. It magnifies a selective part of your scene, like binoculars. Most kit lenses are only slightly telephoto at the longest end, but the longer the focal length of a lens, the narrower its angle of view and the greater its magnification.
Telephotos are ideal for filling the frame with subjects you can’t get close to. They can be used for picking out interesting areas of a landscape scene, and for isolating a portrait subject from their background. Telephotos give the illusion of a shallower depth of field than wideangle lenses, and of compressing distance, making far away subjects seem closer to near subjects.