You do not actually need a lens to take a photograph. A small hole will do the job quite well. Thousands of years ago, it was known that a small hole in the shutter of an otherwise dark room would project an image of the outside view on the opposite wall. Reduce the room to the size of a small box and you have the principles of modern photography. With a tiny hole in the centre of one end of the box, and a recording medium at the opposite end, you can take pictures. ‘Pinhole’ photography, as it is called, is still popular with some photographers today.

Exposure times for pinhole photographs are usually measured in seconds, or even minutes. To reduce exposure times, you need a larger hole. Unfortunately, this quickly throws the image out-of-focus. However, if you place a lens in the larger hole, you can bring the image back into focus and reduce exposure times.

The earliest known lens is nearly 3,000 years old and was found during an excavation of the ancient city of Nineveh, now the site of Mosul, the second largest city in modern Iraq. The lens is made of polished crystal and is about 4cm in diameter. Some modern lenses are still made from crystal, but polished glass is used most of the time.

Early lenses were nothing more than a single piece of glass or crystal. The lens you use on an EOS camera, however, is created from a number of pieces, called elements. The EF70-200mm f/2.8L IS USM lens, for example, contains no less than 23 separate elements. Other lenses, such as the EF28mm f/2.8, contain only five elements.

Some elements in a lens are glued together; others are separated by air. When a lens is described as having, for example, six elements in five groups, it means that two of the elements are glued together to form a single unit.

The lens on all EOS cameras is interchangeable. You can remove one lens and replace it with another. Canon has introduced around 120 EF lenses. Of these, around half are now discontinued, leaving over 60 lenses currently available.

Many years ago, the number of elements was used as a guide to the quality of a lens − more elements were assumed to mean a better lens. Even if this was true then, it is not a good guide today. The use of computers to design all aspects of a lens means that excellent results can be obtained with fewer elements than before.