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Obtaining good exposure for a photograph is achieved by allowing an appropriate amount of light to enter the camera and expose the film -- too much light and the photograph will be overexposed; too little, underexposed. The correct amount of light depends on four factors:
Factor 1 is controlled by the intensity of the light source and whether anything is blocking the light from falling on the subject being photographed. For natural light photography the light source is typically the sun and the photographer is therefore concerned with the intensity of the sun and what may be blocking its light. The intensity of the sun varies with its position in the sky (above or below the horizon) and its light can be blocked from striking the subject by clouds, trees, buildings, etc (ie. the subject is in shadow). The subject itself can even block its own illumination and therefore the position of the sun with respect to the subject may be important -- if the subject is side-lit or back-lit, some of the subject may be in shadow.
Factor 2 is controlled by the reflectivity of the subject. Simply put, if the subject is bright, such as a snow-covered volcano [off-site], it will reflect more light toward the camera than a dark subject, such as pahoehoe lava rock [off-site].
Factor 3 is controlled by the camera settings for shutter speed and aperture. A slower shutter speed and/or a larger aperture allows more light to strike the film.
Factor 4 is controlled by the speed of film used. "Fast" films respond quickly to light while "slow" films respond slowly. For the same photographic scene, a slow film needs more of the scene's light to make a correct exposure than a fast film. Film speed is given by the ISO (or "ASA") film speed number.
Taking all four factors together makes for a very complex situation. Let's simplify. First, note that the photographer only has direct control over factors 3 and 4: the camera settings -- shutter speed and aperture -- and the film speed. Let's assume a film speed of 100 (good grain yet relatively fast). This leaves shutter speed and aperture under direct control. Thus, to make a correct exposure, shutter speed and aperture must be set to "balance out" the two factors over which the photographer has no direct control, factors 1 and 2. That is, for a given film speed, the shutter speed and aperture must be set based on the amount of light incident on the subject and reflected from the subject. In other words, shutter speed and aperture must be set based on the sun intensity, blockage of sunlight, and brightness of the subject for the scene being photographed. How can we measure the factors that are not under the photographer's control in order to select the appropriate shutter speed and aperture?
To assess the light condition of a scene and choose appropriate camera settings, a photographer could simply rely on the light meter and automatic settings of the modern camera. The camera's light meter measures the light reflected from the subject and chooses camera settings for a "correct" exposure. For example, it's midday with a weak sun in a hazy sky (factor 1) and a snow-covered volcano (factor 2) is being photographed. The light meter first measures the amount of light reflected from the subject into the lens, which depends on both factor 1 and 2. Based on this measurement, it then selects a shutter speed and aperture (factor 3) suitable for the film being used (factor 4) and takes the shot. All factors would seem to be accounted for in order to make a good photograph.
But there's a problem. How can the camera possibly "know" that the subject is a volcano covered with bright white snow? It can't. It knows that a high amount of light is entering the camera but it doesn't know whether that light intensity is due to a high light source (factor 1) or a very reflective subject (factor 2). If it doesn't know the reflectivity of the subject, how can it choose the camera settings for a correct exposure? It can't. If it can't choose the correct exposure, how can it take the photo? To choose camera settings the camera has to make the assumption that the subject is not too bright and not too dark -- that the subject reflects 18% of the light falling on it (a medium brightness). Using this assumption it sets the camera's shutter speed and aperture. But the resulting photograph will show a volcano covered with grey snow -- not the correct exposure. The camera does not know that the volcano is covered in white snow.
What if the camera is then pointed at the dark pahoehoe lava flow that recently erupted from the volcano? Again the camera is not aware that it has been moved to a new subject. It simply registers less light and, to compensate, will use a longer shutter speed or larger aperture to allow more light to reach the film. It would do the same if, while still focused on the bright volcano, heavy clouds moved in front of the sun. But the sun intensity has not changed, only the subject reflectivity has changed. In essence, there are too many variables encompassed by factors 1 and 2 for the camera to decide on its settings. To make the settings, it has to reduce the variables by again assuming that the subject is of medium reflectivity. The resulting photograph shows grey, not black, lava.
Obviously, the only time the camera's assumption is correct is when the subject in fact has a medium brightness. If the subject is medium bright (say, the green grass that was spared from the hot lava -- about 18% reflective) and the camera selects a shutter speed and aperture on the assumption that the subject is medium bright, the resulting photograph will have correct exposure. This, then, is the clue to obtaining a correct exposure for subjects that are not medium bright. That is, under a given lighting condition, meter off a subject of medium brightness to select shutter speed and aperture and then use these same settings to photograph subjects of different reflectivity. The following example will illustrate this.
For example, say the camera automatically selects 1/125th of a second at aperture f/22 for the snow-covered volcano (still assuming lighting conditions of a hazy sky and film speed ISO 100). We now know this is wrong and will result in grey snow. To make the snow white, we need more light to expose the film which can be achieved by using a larger aperture1 -- possibly f/16, f/11, or f/8. But which one? Metering off the medium-bright green grass2 we find that the camera selects 1/125th of a second at aperture f/11. Thus, f/11 is the correct aperture for the snow-covered volcano as well. By shooting the volcano at 1/125th and f/11 we are telling the camera what it cannot by itself know -- that the subject is brighter than medium grey and needs more light for a correct exposure.
What about the dark lava? The camera might select 1/125th of a second at aperture f/5.6. We know this will result in grey lava and, to make the lava dark, we need less light to expose the film by using a smaller aperture1. Again metering off the grass2 we get an exposure of 1/125th at f/11. If we shoot the lava at f/11, not f/5.6, less light will expose the film and the lava will be dark -- a correct exposure.
1) This example assumes the camera will automatically adjust its aperture but
it may adjust its shutter speed instead. The concept is the same in either case.
2) In both cases, proper exposure will only be obtained if the green grass is
illuminated under the same incident light as the volcano and lava.
We've just found that the correct exposure for all the subjects in this example -- the snow-covered volcano, the black pahoehoe lava, as well as the green grass -- is 1/125th of a second at aperture f/11. So, does the reflectivity of the subject, factor 2, really matter? Essentially, the reflectivity of the subject is important in the sense that we want to realistically capture the brightness or darkness of the subject in the final photo. Factor 2 definitely affects the amount of light entering the camera lens and therefore affects exposure. But the key is we do not want to negate this effect; we want the reflectivity of the subject to directly influence the exposure in order for the subject to look realistic in the photo. The problem is that the camera's automatic exposure system does negate this effect with its assumption that all subjects are of medium reflectivity, resulting in incorrect exposures for all but medium brightness subjects.
Why are camera controls for shutter speed and aperture even needed? If we don't want to negate the exposure effect of factor 2, why not just shoot everything all the time at 1/125th and f/11? We still need to control the shutter and aperture because we still have not addressed factor 1, the overall light intensity that is illuminating the subject.
To summarize so far, of the four factors affecting exposure, the photographer only has control over the shutter speed and aperture (factor 3), and film speed (factor 4). Assuming a given film speed, the photographer must choose a shutter speed and aperture based on the amount of light incident on the subject (factor 1) and the reflectivity of the subject (factor 2). But as seen above, because we want to realistically portray the brightness or darkness of the subject, the reflectivity of the subject should not really affect the choice of shutter speed and aperture. Thus, the amount of light incident on the subject remains as the primary factor controlling exposure. For correct exposure we must assess this factor and select an appropriate shutter speed and aperture for the photograph.
How do we independently assess factor 1, the amount of incident light? Three approaches are available:
Approach number one has already been discussed. The problem here is that the camera's meter measures the light intensity entering the lens, which depends on both the amount of reflected light from the subject as well as incident light falling on the subject. But the meter can't discern how much light comes from each factor and therefore the camera cannot independently measure the incident light alone. By assuming that the subject is always medium bright, the meter essentially lumps factors 1 and 2 together in order to guess at a proper exposure. Still, this approach can be used, but only if the photographer compensates for the camera's assumption that the light reflected from the subject is medium bright.
Approach number two is sometimes a viable solution. An incident light meter is designed to measure just that -- the light falling on the subject. Its proper use, however, logically requires that the meter be held in the same light as the light in which the subject is illuminated. This is not always possible. The subject may be far away from and under different lighting conditions than the photographer's position. How to measure the incident light on that distant snow-covered volcano while standing in a shady canyon?
Approach number three: gain and use your own experience. As with most everything in life, the best results are the fruit of much trial and error, perspiration, contemplation, and determination. Photography is no exception (sophisticated, multi-segment metering systems notwithstanding). Note that this approach does not preclude using the camera's exposure system, an incident light meter, or anything else. It just means that experience and understanding must accompany their use for the best results. Indeed, given all the other complicating factors -- different film types, limited tonal latitude of film, various metering systems, depth of field and action-stopping considerations, reciprocity failure, etc. -- no other viable approach to proper exposure really exists.
