The emulsion of a film is very thin, but it is still thick enough to be able to produce a wide range of tones in the final image. The more light that strikes a particular spot, the further it penetrates into the emulsion and the more silver halide particles it affects. After processing a negative film, the image is therefore darkest where most light struck the film, and lightest where least light struck it; the light and dark parts of the subject are therefore reproduced as dark and light in the negative. When a negative is printed, the light and dark parts are reversed again, so that the light parts of the original subject appear light in the print.
Film must be given the correct exposure (within its latitude) in order to produce a good negative or slide. If it is overexposed to light, several of the lightest tones in the subject will change the full depth of the emulsion, so the difference between them will be lost, and there will be no detail in the lightest parts of the print or slide. If the film is underexposed, several of the darkest tones will fail to change the emulsion, and there will be no detail in the darkest parts of the print or slide.
Not all films require the same exposure; a slow film needs more exposure than a fast one. Slow films have finer grains composing the image and so can record more detail. Films of different speeds may also have different contrasts, affecting their ability to record a wide range of highlight and shadow detail.
The situation is similar with digital cameras. Instead of a light-sensitive film, there is a CCD or CMOS device that is sensitive to light. Just as with film, the exposure needs to be correct, and overexposure will cause a loss of detail in the brightest areas while underexposure will cause a loss of detail in the darkest areas. With many digital cameras, you can simulate the effect of faster film, but there is always some associated loss of quality.
Computer applications such as Adobe Photoshop and its many competitors can be used to adjust the brightness, contrast and colour of all or part of a digital photograph. However, these techniques take time to learn as well as to apply, and you will always get the best results by getting the original exposure correct.
Correct exposure can either be measured with an exposure meter, or assessed from the results of previous films used under similar conditions. Exposure meters can either be built into the camera, or be separate. They can measure the light from either a small area (spot meters) or a whole subject (integrating meters). Spot meters give the most accurate results, but need much more skill to use, because it is essential to measure the proper part of the subject, which depends on how the meter has been calibrated. Integrating meters are easy to use, but can be fooled by some types of subject, especially when taking photographs through a microscope.
Integrating meters work by producing a blurred image of the subject, so that the light, dark and intermediate tones merge to a uniform grey, and then measuring the brightness of this grey image. Most ordinary subjects can be metered with a high degress of success with an integrating meter, but you must beware of the types of subject that can fool a meter.
If a pinned insect is arranged with a white background and the exposure measured, and then the background is changed to black and the exposure measured again, different readings will be obtained. But the lighting on the essential subject (the insect) has not changed, so the film needs the same exposure for each background, and the meter is misleading you. You have to learn to adjust meter readings to take account of the different types of background. For a light background, give more exposure than the meter says, and for a dark background give less exposure. Extreme examples of light and dark backgrounds occur in photomicrography, for example an insect leg in bright field illumination and a transparent wing in dark field illumination; in both these cases the brightness of the essential subject is very different from the overall brightness. Experience is the best guide to the modifications you must make to the measured exposure.
Exposure is controlled by three factors – the light intensity, the camera shutter speed, and the lens aperture diaphragm.
Light intensity can be controlled by using neutral density filters over the lamps or the lens, by varying the voltage (but this also changes the colour of the light), by changing the output setting of an electronic flash, by using diffusers, or by moving the lamps further from or nearer to the subject.
The camera shutter speed controls the length of time for which the film is exposed to light, a faster speed reduces the exposure, and a slower speed increases it. Shutters are normally marked with settings that halve or double the exposure, and some cameras permit intermediate speeds to be set. To save space, shutter speeds are marked as reciprocals, so that 1 means 1 second, 30 means one thirtieth of a second and 1000 means one thousandth of a second.
Lens aperture diaphragms are marked with a sequence 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45 etc. which also halve or double the exposure; the lowest numbers give the most exposure. Some cameras have intermediate settings marked, and most permit intermediate settings. The numbers represent reciprocal fractions of the focal length of the lens, so for a 50 mm lens, 2 is an aperture of 25 mm. The numbers are called stops or f numbers, and should be written as f/2. As far as the film is concerned, the lens is the source of light, and so exposure is proportional to the area of the lens. The area of the lens is related to the square of its diameter, and squaring the f numbers gives 2, 4, 8, 16, 32, 64 etc., so f/4 gives half the exposure of f/2.8.
Changes in exposure are often referred to as a number of stops, even though it may be light intensity or shutter speed that is actually changed. This dates from the days before iris diaphragms, when the aperture of a lens was reduced by a “stop”, a metal plate with a hole in it. Reducing the aperture with an iris diaphragm is still called stopping down the lens.
In close-up and macro photography, there are two complications that do not occur in normal photography. One of these is reciprocity failure. Over a normal range of shutter speeds (1 sec. to 1/1000 sec.), identical exposures can be achieved by a variety of combinations, for example 1/1000 at f/1.4, 1/60 at f/5.6 or 1 at f/45, by changing the aperture by one stop and halving or doubling the shutter speed. This is called reciprocity. At long exposure times, reciprocity failure occurs progressively, and using the exposure indicated by the meter will result in underexposure. A similar situation occurs with very short exposures, such as provided by some automatic flash guns. Film manufacturers will send you a graph or table showing how to correct for reciprocity failure with their films. With colour films, the three emulsions can show failure at different rates, so that colour balance may also need to be changed by using filters at long exposure times.
The second complication arises from the use of extension tubes, bellows or close-focusing lenses, and stems from the fact that f numbers are only accurate when the lens is focused on infinity, when the distance from the lens to the film is equal to its focal length. No problems arise within the focusing range of a normal lens, or if you use a TTL meter. Problems arise with a separate meter, because as a lens is moved further from the film its effective aperture changes. For example, at life-size reproduction the lens is 2f from the film, so the setting marked f/4 is only transmitting light equivalent to f/8, while at ×10 magnification the lens is 11f from the film, so marked f/4 transmits light equivalent to f/45. The factor for correcting magnification is (m + 1)2, where m is the magnification. Correction can be made by changing any factor that controls exposure, but since you will usually need good depth of field it is normal to change either light intensity or shutter speed.