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make a focusing correction. Follow the detailed instructions of the film manu- facturer.
The position of the infrared index mark has been computed for the use of infrared film with peak sensitivity at 800nm (such as Kodak IR 135) and a red filter such as Wratten 87.
Checking the Depth of Field
There are two ways to find out what the depth of field is. One is by using the depth of field scale which is a series of f/stops repeated on each side of the distance index mark on the lens barrel. The scale differs according to the lens. First focus your subject. Find the two f/stops on the depth of field scale which correspond to the aperture you or the camera have set for the exposure. Draw imaginary lines from these two f/stops to the distance scale. The effective depth of field extends between those two distances. For example, using a standard 50mm lens focused at 3m with the aperture set at f/8, depth of field extends from 2.4m to 4.5m. Any subject from 2.4m to 4.5m away will be in reasonably sharp focus in the image.
You can also visually check the depth of