An eyepiece is an optical system connected to the objective, which can magnify the image coming from the lens or the primary mirror. He straightens the light rays to put them in parallel and provide an image to infinity, which does not require accommodation of the eye.

Constitutive Elements

An eyepiece consists of several sticked lenses, surrounded by a metal cylinder on which is written the focal length (in mm).


The outside diameter of the eyepiece logically corresponds to the inner diameter of the eyepiece holder in which it binds. There are 3 standard connection diameters:

► 24.5 mm (0.9683″): Japanese diameter. 24.5 mm diameter eyepieces are for lower-end instruments and transmit an often dark image and a narrower field of view.
► 31.75 mm (1.25″): American diameter. This is the most common standard. Most of eyepieces on the market have this diameter.
► 50.8 mm (2″): Another american standard. It is less common and used for high-end eyepiece, with long focal length and wide field of view. 50.8mm eyepieces are essentially dedicated to telescope having a diameter greater than 300mm.
(> We also could mention the french standard 27mm and 50mm, but it fell into disuse.)

There are adapters which fix eyepiece whose diameter is different from the the eyepiece holder. For example, there are rings to adapt eyepieces of 24.5mm to 31.75mm eyepiece holders.

Optical Features


The magnification is equal to the focal length of the lens divided by the one of the eyepiece :

Magnification = Objective focal length / Eyepiece focal length


In general, we do not use an eyepiece giving a magnification of more than 2 times the diameter of the lens (in millimeters): 220 times or for a telescope of 110mm, for example.


The focal length of an eyepiece is the distance between the center of the last lens and the point of convergence of light rays: the hearth-image. It is written on the eyepiece in millimeters or inches. The focal length available in the current market range from 3-56 mm. The longer the focal length is, the wider the field of view is. When your eyepiece focal length is too short, the field of view is reduced and the eye relief becomes too small.


The diaphragm is a cache that voluntarily reduces the observed image edges, it is often of lower quality as one moves away from the axis of the eyepiece. This diaphragm is generally in the casting, but can also be inside the eye between two lenses. It depends on the optical design.


This is the maximum cone of vision that can embrace the eye when it is fixed. It is expressed in degrees (°), minutes of arc (‘) and second of arc (‘ ‘). 1 is equivalent to 17.5 m wide at 1000 m distance.

Apparent Field ] It depends only on the eye, its aperture and focal length. this is the field that could see through the eyepiece to a x1 magnification. a telescope eyepiece can provide an apparent field of from 30 ° to 84 °. if it exceeds 65 °, it is said to “large field”. over the apparent field is wide, the better the user experience, especially for objects of great apparent diameter as the moon or nebulae. if the apparent field of the eyepiece is smaller than 40 °, you’ll feel like looking through a keyhole. however, if the eyepiece field is greater than 65 °, you will have the feeling to dive into heaven without experiencing discomfort.

Real Field ] The actual field is the portion of the sky visible through the eyepiece, when it is connected to the optical tube. It therefore depends on both the apparent field of the eyepiece and the magnification of the instrument. Over the magnification and the actual field is reduced (and vice versa).To calculate, simply divide the apparent field of the eyepiece by the magnification of the instrument:

True field = Apparent field of ocular / Magnification



the exit pupil is the diameter of the light beam exiting the eye, where there is the narrowest (thus the more concentrated). the eye of the observer must be placed exactly at the exit pupil to maximize the amount of light entering the eye.

Exit pupil = diameter of the objective / magnification



The eye relief is the distance from the eye lens, which is the closest lens viewer’s eye and the pupil exit. This distance is important for people wearing glasses who need a major backward.
It represents the distance that must be taken into his eye behind the exit lens to get the brightest possible image. The eye relief must be greater than 15 mm for people wearing glasses. For those who do not wear, a distance of more than 10 mm provides good comfort of observation. In general, the higher the focal length is and the greater the eye relief is.

Main Optical Formulas

An eyepiece consists of several separate lenses or not by air spaces (1 to 8 lenses of different shapes and sizes). the field lens is the first lens through which the light while the eye lens is the last. In general, the more eye include lenses and it will be corrected optical aberrations. but in return, the image will be darker. each combination of lenses is the so-called “optical design”. It is often specified on the ring in full or by an abbreviation.













Nagler 1

nagler 1

Nagler 2

nagler 2

The huygens and ramsden (sometimes denoted sr), consisting of two lenses, are the most basic. The kellner, made of 3 lenses are of acceptable quality and are parfais for introductory instruments. The orthoscopic (now virtually absent from the market), plossl erfle and are part of the good eye. and finally, nagler are premium eyepieces. In general, three eyes are enough to observe most celestial phenomena (> 3 x 2 = 6 magnifications with a barlow lens).

binocular head ] They consist of two identical eye which can adjust the distance. inside the body of the binocular head is a prism that separates the two light beam from the instrument. Stereoscopic vision causes a feeling of relief planetary observation, especially since less fatigue seeks both eyes. Current instruments are generally sold with 2 or 3 eyepieces average. It is therefore advisable to supplement or replace them with other, more efficient eye. Binocular heads are suitable for eyeglass wearers.

Glare Treatments

in general, the type of treatment used is registered eye:
► monolayer treaty (treaty, coated)
► multilayer treaty (multiple, multi-treated, MC, multicoated)
► fully-coated (CF) and fully-multi-coated (FMC). Only his statements ensure that all air / glass surfaces are treated.

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