Many optical systems use a reticle—a pattern of fine lines, typically positioned in a focal plane of the system—for purposes of alignment, focusing and/or measurement. Reticles are most commonly used in the eyepieces of sighting devices, such camera viewfinders, telescopes and microscopes.
For example, U.S. Pat. No. 5,130,845, whose disclosure is incorporated herein by reference, describes a real image viewfinder with an objective having movable focusing lenses, a field lens, an eyepiece lens and image inverting optics. The field lens surface is at the focus of the eyepiece lens. A reticle pattern is located on the field lens surface. Since the objective forms a focus at the location of the reticle, observation of the reticle indicates when the objective is focused.