Like all other types of photography, in copying there are various factors that must be considered when calculating exposure. You must consider the nature of the original-its color and brightness, the intensity of the light source, the film speed, the filter factor, and the object-to-image ratio or bellows extension

Color and Brightness

Light-shaded or light-toned originals reflect more light than dark originals. Thus, with the same lighting setup, dark originals require more exposure than light originals. The amount of exposure compensation depends on the darkness or lightness of the original. When TTL (through the lens) metering is read directly from the original, a dark original may require twice the exposure of a standard exposure, and a light original may require less than 50 percent of a standard exposure. You should always use a gray or white card to determine the exposure more accurately.

Intensity of Illumination

Intensity of illumination at the copyboard can be controlled by placing the lamps closer or farther from the copyboard, by using lamps of different light intensity output, or by reducing the intensity of illumination by diffusing the light.

The best method for measuring illumination intensity at the copyboard is with an exposure meter. An exposure meter is particularly useful when the copy lights are moved or changed from the positions used to calculate a standard exposure.

The recommended ISO film speeds or exposure indices of copy films apply directly when an incident meter is used or when a reflected meter reading is taken of an 18 percent gray card at the copyboard. When a gray card is not available, a reflected meter reading of a matte white surface with about 90 percent reflectance can be taken. The back of white photo paper provides this reflectance. When a white surface is used to calculate an exposure, the ISO or exposure index of the film should be divided by 5 and rounded off to the nearest setting on the meter. For example: when the ISO is 32, divide by 5 and use 6 or the nearest setting on the meter. You also may take the meter reading directly without changing the ISO and increase the exposure by two f/stops. Remember, exposure meters are calibrated to produce middle gray regardless of the light reflectance ability of the subject. Thus the light reflectance ability of an original should be considered in determining an exposure.

The exposure indices, given for high contrast materials used in line copy work, are intended for trial exposures, even when an exposure meter is used.

Exposure Compensation for Bellows Extension

As discussed in chapter 4, an exposure calculated with an exposure meter is precise only for a lens set at a distance equal to one focal length When the distance between the optical center of the lens and the focal plane is greater than one focal length, an increase to the indicated exposure is usually required. Before an accurate increase in exposure can be applied by opening the lens diaphragm, the effectiveness of the f/stop of the indicated exposure should be determined Remember, the marked f/stops of a lens that is set beyond one focal length are not valid because the f/stops are a ratio of the diameter of the lens aperture to one focal length. Refer to chapter 4 to determine how to compensate exposure for bellows extension.