EXPOSURE CALCULATION

LEARNING OBJECTIVE: Identify the components used to calculate a photographic exposure.

When you click the shutter, a series of events occur inside the camera. The shutter opens and closes, and light passes through the lens of the camera onto the sensitized emulsion (film), forming a latent image. The emulsion will eventually yield a record of what the camera saw at the moment of exposure. This series of events will yield a satisfactory photograph, in a technical sense, only if the exposure was correct.

You must compute exposure to make sure that the amount of light reaching the sensitive emulsion is sufficient to record the image. Exposure depends on the sensitivity of the photographic emulsion to light and on the brightness reflected by the original subject. Because you usually desire to record the whole range of tones between the brightest and darkest parts of the original scene, you will have to adjust your exposure accordingly.

The same exposure can be given to a certain subject by using various combinations of lens openings and exposure times - a wide opening and short time of exposure may allow the same total amount of light to reach the photographic emulsion as a small opening and along exposure time. At the moment, your consideration of the other factors involved in exposure, such as image movement, depth of field and the use of filters, is unimportant. After you have decided upon the connect total exposure necessary for a given subject at a given time, you can modify the lens opening and shutter speeds later as you desire for specific results.

Incorrect exposures will ruin more of your photographs than any other technical error, yet accurate exposure is relatively simple. By reading and using the exposure guides contained in the film data accompanying your film, you can expect good results most of the time. However, accurate exposures using daylight or tungsten light sources can only be obtained by the connect use of an exposure meter.

An important factor for you to remember is that no light meter, camera, film or manufacturer can guarantee the connect exposures that good photography demands. The only guarantees are your awareness and practice of the exposure theory and practical meter techniques. Good exposure techniques are efficient and simple. Your having the knowledge of exposure techniques frees you from the stumbling block of exposure determination so you can concentrate on taking pictures, and it simplifies the subsequent developing and printing process.

THEORY

The term exposure, while having different meanings at different times, is most often used by photographers to indicate a certain combination of shutter speed and lens aperture. In this case, the shutter speed denotes the length of time the shutter is "open," allowing light to pass through the lens to strike the film.

As stated earlier, various combinations of lens aperture and shutter speed can yield the same exposure. The correct determination of camera exposure is the object of all exposure tables, charts, calculators and meters.

In any given photographic setting a variety of light will be reflected, since the brightness of various objects will reflect varying densities of light. Therefore, the exposure must be adjusted to produce the correct range of densities.

The result of exposure and development of film is very similar in many ways to that of rain falling on a light-colored concrete sidewalk When the rain begins, only a few drops fall. The cement is darkened at only a few spots. As the rain continues, the cement becomes darker and darker, until it is uniformly wet and dark Continued rain will then cease to cause any more changes in the color of the cement sidewalk.

You have experienced differences in the intensity of rain showers. At high intensities, much water comes down in a unit period of time, such as one minute. At low intensities, the amount of water is much smaller. As a result, you could get the same total amount of water within varying periods of time, according to the intensity of the rain. The total amount of rain recorded is equal to its intensity multiplied by the time during which it fell. The effects with light are very similar. Exposure is the amount of light falling on a unit area of the film or on a unit area of photographic paper. The intensity is the amount of light falling on this unit area during the exposure time. Thus the equation for exposure is as follows:

Exposure = Intensity x Time (E = I x T)

Another similarity between light and rain on a sidewalk is in the blackening effect. With light the blackening (during development) increases with the exposure received by the sensitive film emulsion. The photographic lens and shutter assembly should be regarded as a device that controls the camera exposure received by the light-sensitive film emulsion inside the camera.

The aperture of the lens diaphragm controls the intensity of the light, and the shutter controls the time of exposure. Since a photographic reproduction of the original scene contains a range of tones of different brightness, a corresponding range of photographic exposure is given to the sensitized emulsion.