The camera controls

Press, Volume CXII, Issue 32860, 8 March 1972, Page 15

The camera controls

The exposure guides supplied by film manufacturers are simple to follow and give surprisingly accurate results. Many beginners using exposure metres would, in fact, get better negatives and transparencies if they put the meters away and relied on the guides. This does not mean there is anything wrong with using meters if you know how, but they are specialist tools and need to be handled with some skill. Getting the right exposure is like filling a glass with water. Opening the shutter is like turning on the tap, and no matter what the circumstances the same amount of light has to pass through the lens each time you open the shutter, and the same amount of water has to pass through the tap.

If the water pressure is high you will only open the tap partly, and you . will close it quickly. If the photographic subject is brightly lit, you will only open the lens partly and will have the shutter set to close quickly. Aperture Except in the very simplest cameras, the opening of the lens is controlled by a diaphragm consisting of a set of movable leaves which spread out or close together to expose more or less of the full diameter of the lens. Again with the exception of the simplest cameras, the lens is actually a combination of lenses mounted in two groups and the diaphragm is placed between the two groups, and if you look at the lens

while operating the aperture control you will see the leaves opening and closing. The aperture settings are graduated in what- are called “stops,” and for reasons I’ll discuss later the scale has a curious progression. The smallest .figure—say fl .8, or f3.s—represents the widest opening of the lens and is known as full aperture. The next figure will be part of the universal scale which runs: fl, f 1.4, f2, f 2.8, f4> f 5.6, fB, fll, fl 6, f 22 and f 32. Moving the control one step down the scale—e.g. from f 8 to f 5.6 —doubles the exposure, which is also known as increasing it one stop. Obviously, moving from any point on the scale to the next highest figure will halve the exposure.

The shutter controls are relatively simple to understand since we are used to time being split into fractions of a second. With modem cameras most of the range of speeds, if not the whole range, is in steps which halve or double the exposure—e.g., l/30sec, 1/60, 1/125, 1/250, 1/500. It follows from this that an aperture opening of f 5.6 and a shutter speed of 1/250 will give the same exposure (given the same light) as an aperture one stop smaller (f 8 and a shutter speed one step slower (1/125).

Every camera setting is a compromise. The faster the shutter operates, the less chance there is of either the subject or the camera moving during the exposure. But the faster the shutter speed, the more the aperture has to be opened, and this brings difficulties of its own.

The lens gathers in the rays of light reflected from the scene in front of it and projects them on to the film in the camera, but on any one setting only a proportion of these rays are in focus.

Focusing is normally done by moving the lens forward and backward. In its furthest back position—i.e n closest to, the film — the lens is said to be focused on infinity, and with most lenses this means anything from about 100 ft onwards. The main use for thij setting is landscape photography, and even then it has limitations.

Furthest forward, the lens will normally focus on about 3ft or 3Jft, which is mainly used for close-up portraits.

At any setting there is always a zone of acceptable sharpness in front of and behind the point of sharp focus, and this varies according to three factors. The zone, or depth of field, is greatest with a shortfocus lens (e.g., a normal or wide-angle lens on a 35mm camera) set on a small aperture and focused on a distant object Most lenses when used at a medium distance—say 12ft-25ft—-and a medium aperture will have a depth of field of several feet or even several yards. But a studio camera used at about 6ft with a relatively wide aperture will have a depth of field of only a few inches each way.

With a few lenses, such as the extreme wide-angle

optics for 35mm cameras, the depth of field is so great that it never causes any real problems. But in action photography with telephoto lenses it can be difficult—or even impossible—to use a high enough shutter speed to freeze the action and a small enough aperture to be sure of getting the subjects in sharp focus. Next month: A look at perspective.