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ACTINIC VALUE OF EXPOSURE 75
into the habit of lighting for color ; should black-and-white be desired, it is then necessary merely to reduce the exposure level used.
Color reversal as original film is to be preferred to black-and-white reversal because of the more precise control exercised in developing ; this practical advantage far outweighs the possible theoretical advantage of somewhat better grain characteristics. The graininess advantage of reversal is likely to remain unrealized commercially because of the far wider tolerances customary in black-and-white reversal developing and because of the large haphazard nonprofessional interest that still exists.
Another advantage of photographing the original with color reversal film is that much subject matter has additional significance when seen in color. Once the color original exists, it is possible to make a small percentage of release prints in color if desired, while the remaining prints can be made in black-and-white.
As one example, a picture on first aid may be used in black-and-white for general study, but in a limited number of cases where greater realism is needed for clinical study, color prints may be used.
When photographing the original with black-and-white reversal film, uniform actinic quality is still desirable, since variations in color temperature of the lights give rise to variations in contrast of the original film because of the variations in film gamma with actinic quality of the exposure. Another important consideration also is the fact that exposure meters do not register equal amounts of all colors equally. The effective exposure on the film can vary rather widely in that the color sensitivity of the film may not match the color sensitivity of the exposure meter. New meters better adapted for color photographing will doubtless be marketed in the near future.
"When lamps, receptacles, and cables are made up for use in indoor photographing, it is always good practice to provide at each fixture or lamp a test outlet into which a reliable test voltmeter may be plugged for testing the voltage across the lamp. If the difference between the rated lamp voltage and the voltage as indicated by a reliable meter is greater than 3 to 5 volts with all lamps turned on, steps should be taken to correct the difficulty before photographing is attempted, since serious disturbances of the color balance will otherwise result. The adjustment can be made by means of a voltage regulator or an adjustable transformer such as a Variac. Tn measuring the voltage, the meter used should be a rugged long-scale instrument (7 inches or so) with accuracy guaranteed by the manufacturer in the order of 2%. The full-scale