The distance in front of and behind a sharply focussed subject in which details also look sharp in the final image is called depth of field. It occurs because the eye cannot distinguish between a circle smaller than a particular limiting size and a sharply focussed point in an image. Thus, within the camera, cones of light rays from subject points at various distances do not converge exactly on the image plane ( i.e. the film plane). If they form circles sufficiently small as they cross the film plane, they will appear equally sharp. Anything that causes the cone angles to be smaller at the image plane increases the depth of field at the subject.
In practice, this is accomplished by setting the lens aperture to a smaller f-stop (e.g. moving the aperture from f5.6 to f11), by moving away from the subject, or by changing the lens to one of shorter focal length; a combination of two or all three procedures will further increase depth of field. Conversely, depth of field is decreased by using a larger f-stop, a longer focal length lens, or a position closer to the subject.
Some lenses will have a depth of field scale engraved on the lens barrel which will give you an indication of the amount of depth of field available at each lens stop. Some cameras also provide a depth of field preview lever or button which can manually stop down the lens to enable the depth of field to be observed in the viewfinder. However, small apertures may reduce the light to such an extent that seeing the amount of depth of field in the image when looking in the viewfinder may be difficult. Many "point and shoot" cameras use a slightly wide angle lens (around 35 to 40mm focal length) to ensure that users will have images that are reasonably sharp from having enough depth of field.
Understanding depth of field is important because it gives you the ability to isolate your subject from other parts of an image which may be distracting or to which you may wish to give a special impact. It also ensures that everything that is in an image will be sharp if it is supposed to be!
A film emulsion is composed of very small grains of silver halides which are sensitive to light and are adhered to the film base by gelatin. Light falling on the silver grains causes a chemical change that is proportional to the intensity of and the total amount of light received. This can be summed up by the formula Exposure = Illumination x Time. In a camera, control of the parameters illumination and time are obtained by means of the aperture (or diaphragm) and the shutter respectively. The diaphragm controls the intensity of the light passing through the lens on its way to the film, and the shutter sets the length of time this will occur.
The formula indicates that many different values of illuminance and time can produce the same exposure. However, there is now a standard system built into every camera which relates these two variables.
The aperture is usually marked by a series of numbers such as 1.4, 1.8, 2, 2.8, 4, 5.6, 8, 11, etc. These numbers represent settings of the diaphragm, referred to as stops or f-stops. The difference between each stop represents an increase or decrease (depending upon whether there is an increase in f-number or a decrease) of twice the amount or half the amount of light admitted by the diaphragm. Similarly the shutter speeds follow a sequence that doubles or halves the time the shutter is open e.g. 1/15th sec, 1/30th sec, 1/60th sec, etc. In practice, this means that a shutter and aperture setting of 1/250th sec and f5.6 will produce the same exposure as settings of 1/60th sec and f11.
If control of depth of field is important for a photograph, then a small aperture will probably be required. (This may be true for say, landscape photography where great depth of field may be imperative). However, shutter speed may be a priority, such as in recording sports action, and therefore the aperture setting is probably less important, so wider aperture settings are generally used.
Whilst most modern cameras have an exposure meter included as part of their design and are usually quite accurate, it is important to remember that they are not infallible. The modern meter may be one of two designs. Most SLRs will measure exposure by sampling the light actually passing through the lens system. Compact cameras usually measure the light passing through the viewfinder. Most SLRs have meters that are "centre-weighted" which means that they measure more of the light coming from the lower centre of the image - the reasoning being that the main subject will be within that area and this also avoids the light from the sky (being usually the brightest area in a scene) affecting the reading. Other SLRs offer more complicated arrangements such as "spot" metering and "matrix" metering.
However, these systems have one thing in common in the way they are calibrated: it is a photographic maxim that the "average scene" has a light value that it produces on a black and white print known as "18% grey". That is, the average reflectance from an average scene is equivalent to the amount of light reflected from a grey card which reflects 18% of the light falling on it. This may or may not be true, but what it means in practice is that such a meter will give an erroneous reading if the scene to be photographed varies considerably from that average amount. For example, say we have a white wall against which we want to photograph a black cat, the white wall dominating the scene. The meter reading from an exposure meter will indicate that the white wall reflects light equivalent to 18% grey and the camera will select an exposure which will underexpose the film. On the other hand, say the condition is reversed and we have an all black fence with a white cat. In this case, the meter will "see" the fence as 18% grey and will overexpose, rendering the fence a grey colour rather than black.
The above means that normally you will be able to trust that the exposure meter in your camera will give a "correct" exposure, but you will also need to be aware that with scenes of more extreme contrast, you may need to vary the meter's reading to accommodate. This means giving either more or less exposure than indicated by the exposure meter, either by opening/closing the aperture, increasing or decreasing the shutter speed, or using the "exposure compensation" control on your camera (if it has one).