Eye relief

Eye Relief and Exit Pupil

The eye relief of a telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

, a microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

, or binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

 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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

 at which the 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 shorter the eye relief.

The eye relief property should not be confused with the exit pupil
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 compound microscope, this image is the image of the...

 width of the product, 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 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.
Eye relief distance can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the 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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

 so needs a longer eye relief in order to still see the entire field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....

. 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. An optic
Telescopic sight
A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...

 with too short an eye relief can cause a skin cut at the contact point between the optic and the eyebrow of the shooter during recoil
Recoil is the backward momentum of a gun when it is discharged. In technical terms, the recoil caused by the gun exactly balances the forward momentum of the projectile and exhaust gasses, according to Newton's third law...

. This is frequently called the "idiot cut" due to the obvious and long-lasting nature of such a mistake. Typical eye relief distances for telescopic sights are typically 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 him whether he prefers his 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 your 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 required eye relief that he requires, it should be noted that some cameras and microscopes allow prescription lenses to be fitted onto their eyepieces. In this way the user can temporarily dispense with his glasses in favor of the lens mounted on the optics. Although not ideal, in that this method does not afford good incidental vision for the field around him, (only through the optics), it might still be of use to some individuals.

Further reading

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.