Kodak ISBN 0-87985-749-8 manual Plus-X Double-X Tri-X

	Filter Factors* for KODAK Motion Picture Films
	Negative Films		Reversal Films
Filter	PLUS-X	DOUBLE-X	PLUS-X	TRI-X
No. 3	1.5	1.5	1.5	1.5
No. 8 (K2)	2.0	1.5	2.0	2.0
No. 12	2.5	2.0	2.0	2.0
(Minus Blue)
No. 15 (G)	3.0	3.0	2.5	2.5
No. 21	3.5	3.0	3.0	3.0
No. 23A	5	5	5	5
No. 8N5	5	5	6	6
No. 25	8	8	10	10
No. 29	25	20	40	40
No. 96	8	8	8	8

*All filters absorb part of the incident radiation, so their use usually requires some increase in exposure over that required when no filter is used. The number of times by which an exposure must be increased for a given filter with a given mate- rial is called the filter factor, or multiplying factor.

Filter factors published for Kodak products by Kodak are intended only as approximate guides. Published filter factors apply strictly to the particular light- ing conditions used in the laboratory where the factors were determined. For scientific applications, especially, the quality of light can vary widely so that it may be desir- able to determine the filter factor for actual working conditions.

To determine a filter factor, choose a subject having a neutral-gray area, or place an 18-percent gray card or a photographic gray scale in the scene to be photographed. Make one exposure without a filter. Then, with the filter where it will be for the exposure, and beginning with the unfiltered exposure setting, make a series of exposures. Increase each in 1⁄2-stop increments through a 2- to 4-stop greater exposure (very dense filters may need more). After processing the negative, match densities of the unfiltered exposure with the filter series either visually or with a densitometer. Choose the filtered exposure that is closest to the unfiltered. Some additional exposure adjustments may be necessary.

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