Interchangeable Camera Lenses
Have you ever watched a magnifying glass concentrate the rays of the sun to burn a hole in a piece of paper? Then you have also witnessed the primary function of all photographic lenses – to bend, or refract, light. We use this ability of lenses to translate the three-dimensional real world into a two-dimensional representation on film. Lenses employed in modern cameras are far more complex than the simple magnifying glass. They are designed by computer and use combinations of several elements to eliminate distortion and colour fringing, or aberrations, and achieve optimum performance.
All lenses have a focal length -the distance from the centre of the lens to the point where focus is achieved. With the simple magnifying glass it is the distance between the lens and the paper which causes smouldering – all the sun’s rays converge on one tiny spot. In a modern camera optic, the lens barrel holds the glass elements at the focal length from the film plane.
Compact cameras have increased in versatility in recent years with the advent of dual lens and zoom models but the only way to explore broader optical horizons is via the SLR route. With a single lens reflex the viewing of the subject takes place through the taking lens giving optimum accuracy and the ability to view the scene through whatever lens is attached to the camera.
As the SLR has become more sophisticated, so the range of lenses to fit it has grown also. Today most manufacturers produce a wide choice of optics, and some (Nikon, Minolta, Canon and Pentax) have created lens systems of extraordinary range.
Why do we need so many different lenses.’ With them, photographers can tackle every subject and produce every effect they want to achieve.
Different focal lengths enable a photographer to adjust the size of a subject on film. A 200mm lens will produce an image four times the size of that produced by a 50mm. However the area of subject placed across the film format is reduced, because the telephoto lens also has a narrower angle of view. A lens of 1000mm, then, magnifies the same subject some twenty times, but records only a small part of it.
When the focal length of a lens is shorter than standard, the opposite occurs. When compared with a standard lens, a 28mm optic reduces the scale of everything in the shot, but there’s more included, because the lens’s angle of view is wider. At 20mm focal length, the scale of detail is smaller still, but a very wide angle of view is provided. For this reason, lenses shorter than 50mm are termed wide-angles. Those longer than 50mm are called long-focus or telephoto lenses.
AH lenses have a variable iris, or diaphragm, CO control the amount of light entering the camera. An aperture, or f number, is an accepted value of light, with settings which run as follows: f1, f1.4, f2, f2.8, f4 f5.6, f8, f11, f16, f22, f32, f45 and f64. This series of calibrated numbers is, in fact, a strict geometrical progression, with f1 being the widest iris opening and the other figures representing its gradual closure. As the aperture number rises from one figure to the next, the amount of light passing through the iris is precisely halved.
So an aperture of f8, for example, lets in 50% of the light passed at the f5.6 setting. Such a change is also termed one f-stop difference – a measurement of light we’ve already come across in the discussion about exposure control. Adjusting either the shutter speed or the aperture by one step will have exactly the same effect on light reaching the film. Control of these two settings enables metering systems to establish the right exposure on film accurately.