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Digital Electronic Cameras, basic knowledge and care the Lens

The lens illustrated is the type usually fitted to an SLR camera. It encompasses more or less the same view you see with one eye closed. The information on the front of the lens gives its focal length (50mm), maximum aperture (f2), maker’s name, brand name, and serial number. The side view shows scales for focusing, aperture setting and depth of field.

Modern lenses are made up from several glass elements ground to a precise shape and mounted inside a tube.

The 110 camera usually has a lens with a similar view to the 50mm lens above but the focal length is shorter because of the film’s small size.

The lens focuses light on to the film to form an image, and unless that image is sharp you will never produce clear pictures. So it is important to understand how the lens works, and what your particular lens can do. Only when you feel comfortable with the lens controls and can adjust them quickly and accurately will you get the most enjoyment and satisfaction out of trying out different effects in photography.

Focal length

The amount of a scene photographed by any lens is governed by its focal length. On a 35mm camera a normal lens of 4050mm has roughly the same view as the human eye. To photograph more of the subject, or to pull distant objects closer, you need lenses with different focal lengths. The shorter focal length (under 35mm) of the wide angle lens gives a broad view of the scene, while the much longer focal length of a telephoto lens shows up detail in distant objects. Focal length is the distance between the lens and the film when the lens is focused at infinity (for a normal lens, that means set for objects more than 30m away). You can set fire to a piece of paper by holding a magnifying glass at exactly the point where a sharp image of the sun (at infinity) forms on the paper, and a camera lens focuses an image on film in the same way. In each case, the distance between the lens and the image is the focal length.

Now imagine a magnifying glass forming the image of a candle flame on a sheet of paper. The distance between paper and glass before a sharp image is formed is greater than when the sun’s rays are used. The closer an object is to the camera, the further the lens must be from the film to focus correctly. Most simple 110 and 126 cameras have lenses with no focusing adjustment. These have been factory set to keep everything sharp between 2m and infinity. Most 35mm cameras have lenses which must be accurately focused by the photographer. These are set in helical mounts, two telescoping tubes with a screw thread between them. The closest focusing distance is determined by the maximum movement of this mount: for example, the shortest distance a 50mm lens fitted to a 35mm reflex camera focuses at is usually about 30-50cm.

If you have a reflex camera, try this yourself by setting the lens to minimum distance and moving backwards and forwards until an object close by becomes sharp in the viewing screen. Then measure the distance in between.

Normal lenses

The standard lens fitted to a camera is referred to as normal because, like the human eye, it takes in a 45-50° section of the scene. The focal length is usually more or less equal to the diagonal of the negative. Negative size varies, so the focal length of a normal lens depends on the film size a camera takes. In a 35mm camera the normal or standard lens is between 40-50mm, but for a small pocket camera taking 110 film it need only be 25mm.

The larger format cameras have correspondingly longer focal-length normal or standard lenses.

Brightness control

The brightness of the light reaching the film is controlled by a hole called the aperture. It works like the pupil of the eye, large in the dark to let in more light and small in bright conditions. The camera lens controls its aperture with a diaphragm (a mechanical iris) which is operated by a ring or lever outside the lens. The dimmer the light the larger the aperture must be to keep the brightness of the light on the film constant.

f numbers

Simple cameras often mark the aperture control with weather symbols, but the more complex cameras use a numerical system called f numbers. The smaller the f number, the larger the aperture; for example, f2.8 lets in more light than f5.6 and much more than f16.

Depth of field

Changing the f number also affects the depth of field, or amount of the scene in front of the camera that appears in focus. The depth of field scale on the lens indicates how much of the area in front of and behind the subject is in focus. If you move the aperture ring on the lens through the range of f numbers you can see how this control over sharpness can be used to subdue unwanted background and emphasize the main subject or, at a smaller aperture, to take a picture which is sharp from foreground to infinity.

Caring for your lens

• Never touch the lens surface. Greasy fingermarks are hard to remove.
• Use a soft lens brush to remove dust particles from the lens, or polish lightly with a rolled up, lint-free cloth, moving from rim to centre.
• Never clean your lens with solvent. If absolutely necessary; flood the glass surface with distilled water. invert the lens, and blot dry carefully.
• If you drop the lens in water, sand. or mud, dry it gently and take it to a camera shop for cleaning. Do not dismantle it yourself.
• Always replace the case or lens cap after use. When changing lenses, put the rear cap on and look after your lens mount fitting.
• Never oil the moving parts of your lens.
• A skylight or UV/haze filter kept permanently on the lens is a sensible protection against dust particles.
• Always keep your lenses in a cool. dry place.

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