Index marks at the center point of each side of an air negative or photo. These are usually four in number, are rigidly connected with the camera lens through the camera body—which forms images on the negative—and usually define the principal point of the photograph. These marks may take one of various forms; the more usual kinds are a hairline, a cross, or a half-arrowhead. The center, or the principal point, of a print is at the intersection of the two lines drawn between opposite collimating marks. Collimating marks are also marks in any instrument that marks the axes whose intersection fixes the principal point of a photograph and fulfills the requirements of interior orientation. These are also usually four in number. Also called fiducial marks.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved