Lens tips

Basically the type of lens is mostly determined by its size, or focal length. The greater the focal length, the more magnification the lens provides, which in turn affects the field of view and the perspective. In a digital camera with a normal sensor crop factor of 1.6, a focal length of around 30-35mm gives roughly the same field of view and perspective as the human eye. Anything over this is usually known as a telephoto lens.

Once the focal length gets below this, things get weird, objects appear further away rather than closer and at extremely short focal lengths become distorted. Makes portraits interesting though. Another interesting effect of focal length is that as it decreases (wider angle), the depth of field increases, so F8 would give greater depth of field on a 18mm lens than it would on a 70mm lens.

Why have a wide angle, standard and telephoto lens when you can just have a zoom to cover all three? Well, there are two reasons. The first is quality. A zoom has to do more things so it contains more glass, more glass means more interference between the subject and the sensor, reducing the quality. More glass also means less light gets through, which is the second problem. Fixed length, or prime lenses, are able to offer greater maximum apertures, which is a bonus in low light. For example if Simon were to use his 24-85 zoom in low light at its maximum aperture (f/2.8) he may not be able to achieve a fast enough shutter speed to prevent camera shake, if he switched to his 50mm he could then use the f/1.8 aperture and double his shutter speed.

There is also more glass involved in really long length lenses (300mm plus), with the same problems, however, as there is a specific need for these types of lenses (such as sports or nature photography - capturing fast moving subjects from a distance) the manufacturers have overcome these problems, but at a high price - £6000 for a new lens anyone? (Canon 600mm F4 USM).

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