{{Short description, Optical instrument The eye relief of an

optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common examples include periscopes, microscopes ...

(such as a

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...

, a microscope, or

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes ( binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

) is the distance from the last surface of an

eyepiece 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 the device. Th ...

within which the user's eye can obtain the full viewing angle. If a viewer's eye is outside this distance, a reduced field of view will be obtained. The calculation of eye relief is complex, though generally, the higher the magnification and the larger the intended

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Humans a ...

, the shorter the eye relief.

Eye relief and exit pupil

The eye relief property should not be confused with the

exit pupil In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. In a telescope or compou ...

width of an instrument: that is best described as the ''width of the cone of light'' that is available to the viewer at the exact ''eye relief'' distance. An exit pupil larger than the observer's pupil wastes some light, but allows for some fumbling in side-to-side movement without

vignetting In photography and optics, vignetting is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as ''vine'', originally referred to a decorative borde ...

or clipping. Conversely, an exit pupil smaller than the eye's pupil will have all of its available light used, but since it cannot tolerate much side-to-side error in eye alignment, will often result in a vignetted or clipped image. The exit pupil width of say, a binocular, can be calculated as the objective diameter divided by the magnification, and gives the width of the exit cone of light in the same dimensions as the objective. For example, a 10 × 42 binocular has a 4.2 mm wide exit cone, and fairly comfortable for general use, whereas doubling the magnification with a zoom feature to 20 × results in a much more critical 2.1 mm exit cone. Exitpupil

Eye relief distance can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the

, so that user needs a longer eye relief in order to still see the entire

. A simple practical test as to whether or not spectacles limit the field of view can be conducted by viewing first ''without'' spectacles and then again ''with'' them. Ideally there should be no difference in the field. For a shooter, eye relief is also a safety consideration. If the eye relief of a

telescopic sight A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a ''reticle'' – mounted in a focally appropriate po ...

is too short, leaving the eye close to the sight, the firearm's recoil can force the optic's eyepiece to hit and cut into the skin around the shooter's eye, leaving a curved scarring

laceration A wound is a rapid onset of injury that involves lacerated or punctured skin (an ''open'' wound), or a contusion (a ''closed'' wound) from blunt force trauma or compression. In pathology, a ''wound'' is an acute injury that damages the epider ...

on the medial end of the

supraorbital ridge The brow ridge, or supraorbital ridge known as superciliary arch in medicine, is a bony ridge located above the eye sockets of all primates. In humans, the eyebrows are located on their lower margin. Structure The brow ridge is a nodule or crest ...

and the

eyebrow An eyebrow is an area of short hairs above each eye that follows the shape of the lower margin of the brow ridges of some mammals. In humans, eyebrows serve two main functions: first, communication through facial expression, and second, prevent ...

. This is frequently called a "scope bite", or the "idiot cut", due to the obvious and long-lasting nature of such a mistake. Typical eye relief distances for telescopic sights are often between one and four inches (25 to 100 mm), as opposed to the much shorter 15 to 17 mm for typical binoculars. The exit pupil widths in rifle sights are designed to be larger than the eye's pupil, to allow for a range of motion without vignetting.

Available eye relief

The eye relief given in product specifications does not always give a realistic view of what a user can expect. Although eye-cups can usually be folded down to allow the spectacle wearer to get closer to binocular eyepieces, there are sometimes lens mountings that do not allow the theoretical eye relief to be obtained. A better measure for those with strict needs would be one that takes account of this ''available'' eye relief, the theoretical value less any thickness of the lens' rims. This point can account for confusion in performance and is rarely expressed clearly. Additionally, when a spectacle wearer orders new glasses, the optician will ask them whether they prefer their spectacles close to the eyes or at some distance. This distance is referred to as the '' Back Vertex Distance'', or BVD on a prescription. Since this property affects the ''available eye relief'' of any binocular or other optics used, (telescopes, microscopes, etc.) it should be borne in mind at the eye testing stage. The matter should be discussed with the optician, though the only realistic way of testing the comfort is to try the optical device while wearing the usual spectacles. The optician can however make sure that the BVD is no worse in the new glasses than in the old ones that were used during evaluation.

Adding prescription lenses

In the event that a spectacle wearer cannot obtain the eye relief that they require, some cameras and microscopes allow prescription lenses to be fitted onto their eyepieces. In this way, the user can temporarily dispense with glasses in favor of the lens mounted on the optics. Although this method does not afford good incidental vision for the field around them, it might still be of use to some. Optics