A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscope, to provide measurement references during visual inspections. Today, engraved lines or embedded fibers may be replaced by a digital image superimposed on a screen or eyepiece. Both terms may be used to describe any set of patterns used for aiding visual measurements and calibrations, but in modern use reticle is most commonly used for weapon sights, while graticule is more widely used for non-weapon measuring instruments such as oscilloscope display, astronomic telescopes, microscopes and slides, surveying instruments and other similar devices.
Reticle of Bell & Howell Pocket Comparator
Reticle accessory (PD-8) used in sniper rifles
The day and low-light reticles of the integral reflex sight used on the FN P90 / PS90 USG models
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through an optical device to observe an object or sample. The objective lens or mirror collects light from an object or sample and brings it to focus creating an image of the object. The eyepiece is placed near the focal point of the objective to magnify this image to the eyes. The amount of magnification depends on the focal length of the eyepiece.
A collection of different types of eyepieces.
A 25 mm Kellner eyepiece
Simulation of views through a telescope using different eyepieces. The center image uses an eyepiece of the same focal length as the one on the left, but has a wider apparent field of view giving a larger image that shows more area. The image on the right has the same apparent field of view as the center eyepiece but has a shorter focal length, giving the same true field of view as the left image but at higher magnification.
The Plössl, an eyepiece with a large apparent field of view