Let us look at a telephoto lens as an example of a multi-element system. In a 35 mm camera a telephoto lens of 300 mm focal length can in principle be made with a single 300 mm focal length lens, as sketched in the top diagram. For shooting objects at a large distance, what is the film-lens separation? A more compact arrangement may be constructed using a pair of lenses, as shown in the lower part of the diagram above.
As far as the film is concerned, it "sees" light coming in at exactly the same angle and perspective in the two cases, but the second arrangement is much easier for the photographer to carry and hold steady. This is also the type of arrangement used in opera glasses.
Multiple-lens systems allow another problem to be reduced. The lenses are made of glass, and we know that the refractive index of this material varies with wavelength, and therefore so will the focal length of the lens. Thus the red components of an image may be in focus, but the blue bits will not. This is known as chormatic aberration. By careful choice of the shapes and glass-types used in a set of lenses, a compound lens can be constructed where the effects of chromatic aberration are very small.
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