The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece 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 shorter the eye relief.
The eye relief property should not be confused with the exit pupil 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 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 4.2mm 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 2.1mm exit cone.
Optics showing eye relief and exit pupil 1 Real image2 Field diaphragm 3 Eye relief 4 Exit pupil
Eye relief distance can be particularly important for eyeglass wearers and shooters. The eye of an eyeglass wearer is typically further from the eyepiece, so that user needs a longer eye relief in order to still see the entire field of view. 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 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 on the medial end of the supraorbital ridge and the eyebrow. 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 100mm), as opposed to the much shorter 15 to 17mm 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.
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Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms that rest over the ears.
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 when viewing distant objects. Most binoculars are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal-mounted military models.
The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microscope and were possibly invented in their present compound form in the 17th century. Basic optical microscopes can be very simple, although many complex designs aim to improve resolution and sample contrast.
A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a monocular are typically less than half of a pair of binoculars with similar optical properties, making it more portable and also less expensive. This is because binoculars are essentially a pair of monoculars packed together — one for each eye. As a result, monoculars only produce two-dimensional images, while binoculars can use two parallaxed images to produce binocular vision, which allows stereopsis and depth perception.
An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.
Tunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision.
In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, binoculars, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.
An optician, or dispensing optician, is a technical practitioner who designs, fits and dispenses lenses for the correction of a person's vision. Opticians determine the specifications of various ophthalmic appliances that will give the necessary correction to a person's eyesight. Some registered or licensed opticians also design and fit special appliances to correct cosmetic, traumatic or anatomical defects. These devices are called shells or artificial eyes. Other registered or licensed opticians manufacture lenses to their own specifications and design and manufacture spectacle frames and other devices.
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in size, sometimes called minification or de-magnification.
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. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.
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 position in its optical system to provide an accurate point of aim. Telescopic sights are used with all types of systems that require magnification in addition to reliable visual aiming, as opposed to non-magnifying iron sights, reflector (reflex) sights, holographic sights or laser sights, and are most commonly found on long-barrel firearms, particularly rifles, usually via a scope mount. The optical components may be combined with optoelectronics to add night vision or smart device features.
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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 objective element(s) as produced by the eyepiece. The size and shape of this disc is crucial to the instrument's performance, because the observer's eye can see light only if it passes through this tiny aperture. The term exit pupil is also sometimes used to refer to the diameter of the virtual aperture. Older literature on optics sometimes refers to the exit pupil as the Ramsden disc, named after English instrument-maker Jesse Ramsden.
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Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian. If the ocular image size in both eyes are equal, the condition is known as iseikonia.
A loupe is a simple, small magnification device used to see small details more closely. They generally have higher magnification than a magnifying glass, and are designed to be held or worn close to the eye. A loupe does not have an attached handle, and its focusing lens(es) are contained in an opaque cylinder or cone. On some loupes this cylinder folds into an enclosing housing that protects the lenses when not in use.
A digital microscope is a variation of a traditional optical microscope that uses optics and a digital camera to output an image to a monitor, sometimes by means of software running on a computer. A digital microscope often has its own in-built LED light source, and differs from an optical microscope in that there is no provision to observe the sample directly through an eyepiece. Since the image is focused on the digital circuit, the entire system is designed for the monitor image. The optics for the human eye are omitted.
The stereo, stereoscopic or dissecting microscope is an optical microscope variant designed for low magnification observation of a sample, typically using light reflected from the surface of an object rather than transmitted through it. The instrument uses two separate optical paths with two objectives and eyepieces to provide slightly different viewing angles to the left and right eyes. This arrangement produces a three-dimensional visualization of the sample being examined. Stereomicroscopy overlaps macrophotography for recording and examining solid samples with complex surface topography, where a three-dimensional view is needed for analyzing the detail.
Eye care professionals use prism correction as a component of some eyeglass prescriptions. A lens which includes some amount of prism correction will displace the viewed image horizontally, vertically, or a combination of both directions. The most common application for this is the treatment of strabismus. By moving the image in front of the deviated eye, double vision can be avoided and comfortable binocular vision can be achieved. Other applications include yoked prism where the image is shifted an equal amount in each eye. This is useful when someone has a visual field defect on the same side of each eye. Individuals with nystagmus, Duane's retraction syndrome, 4th Nerve Palsy, and other eye movement disorders experience an improvement in their symptoms when they turn or tilt their head. Yoked prism can move the image away from primary gaze without the need for a constant head tilt or turn.
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