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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Eyepiece | 4/7 | https://en.wikipedia.org/wiki/Eyepiece | reference | science, encyclopedia | 2026-05-05T09:47:24.682125+00:00 | kb-cron |
This formula also indicates that, for an eyepiece design with a given apparent field of view, the barrel diameter will determine the maximum focal length possible for that eyepiece, as no field stop can be larger than the barrel itself. For example, a Plössl with 45° apparent field of view in a 1.25 inch barrel would yield a maximum focal length of 35 mm. Anything longer requires larger barrel or the view is restricted by the edge, effectively making the field of view less than 45°.
=== Barrel diameter === Eyepieces for telescopes and microscopes are usually interchanged to increase or decrease the magnification, and to enable the user to select a type with certain performance characteristics. To allow this, eyepieces come in standardized "Barrel diameters".
==== Telescope eyepieces ====
There are six standard barrel diameters for telescopes. The barrel sizes (usually expressed in inches) are:
0.965 inch (24.5 mm) – This is the smallest standard barrel diameter and is usually found in retail toy store and shopping mall telescopes. Many of these eyepieces that come with such telescopes are plastic, and some even have plastic lenses. High-end telescope eyepieces with this barrel size are no longer manufactured, but you can still purchase Kellner types. 1.25 inch (31.75 mm) – This is the most popular telescope eyepiece barrel diameter. The practical upper limit on focal lengths for eyepieces with 1.25″ barrels is about 32 mm. With longer focal lengths, the edges of the eyepiece barrel intrude into the view, limiting its size. With focal lengths longer than 32 mm, the available field of view falls below 50°, which most amateurs consider to be the minimum acceptable width. These barrel sizes are threaded for 30 mm filters. 2 inch (50.8 mm) – The larger barrel size in 2″ eyepieces helps alleviate the limit on focal lengths; it is the largest size commonly available. The upper limit of focal length with 2″ eyepieces is about 55 mm. The trade-off is that these eyepieces are usually more expensive, will not fit in some telescopes, and may be heavy enough to tip the telescope. These barrel sizes are threaded for 48 mm filters (or rarely 49 mm). 2.7 inch (68.58 mm) – 2.7″ eyepieces are only made by a few manufacturers. They allow for slightly larger fields of view. Many high-end focusers now accept these eyepieces. 3 inch (76.2 mm) – The even larger barrel size in 3″ eyepieces allows for extreme focal lengths and over 120° field of view eyepieces. The disadvantages are that these eyepieces are somewhat rare, extremely expensive, up to 5 lbs in weight, and that only a few telescopes have focusers large enough to accept them. Their huge weight causes balancing issues in Schmidt-Cassegrains under 10 inches, refractors under 5 inches, and reflectors under 16 inches. Also, due to their large field stops, without large-diameter secondary mirrors, most reflectors and Schmidt-Cassegrains will have severe vignetting with these eyepieces. 4 inch (102 mm) – Eyepieces this size are rare, and only commonly used for long refracting telescopes in older observatories. Very few manufacturers make them, and with the current popularity of short focal length / smaller focal ratio telescopes among amateurs, the demand for this size is low. They are sometimes improvised from re‑adapted lenses scavenged out of old cinema projectors.
==== Microscope eyepieces ==== Eyepieces for microscopes have a variety of barrel diameters, usually given in millimeters, such as 23.2 mm and 30 mm.
=== Eye relief ===
The eye needs to be held at a certain distance behind the eye lens of an eyepiece to see images properly through it. This distance is called the eye relief. A larger eye relief means that the optimum position is farther from the eyepiece, making it easier to view an image. However, if the eye relief is too large it can be uncomfortable to hold the eye in the correct position for an extended period of time, for which reason some eyepieces with long eye relief have cups behind the eye lens to aid the observer in maintaining the correct observing position. The eye pupil should coincide with the exit pupil, the image of the entrance pupil, which in the case of an astronomical telescope corresponds to the object glass. Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. Until recently, and still quite commonly, eyepieces of a short-focal length have had a short eye relief. Good design guidelines suggest a minimum of 5–6 mm to accommodate the eyelashes of the observer to avoid discomfort. Modern designs with many lens elements, however, can correct for this, and viewing at high power becomes more comfortable. This is especially the case for spectacle wearers, who may need up to 20 mm of eye relief to accommodate their glasses.
== Designs == Technology has developed over time and there are a variety of eyepiece designs for use with telescopes, microscopes, gun-sights, and other devices. Some of these designs are described in more detail below.
=== Negative lens or "Galilean" ===
The simple negative lens placed before the focus of the objective has the advantage of presenting an erect image but with limited field of view better suited to low magnification. It is suspected this type of lens was used in some of the first refracting telescopes that appeared in the Netherlands in about 1608. It was also used in Galileo Galilei's 1609 telescope design which gave this type of eyepiece arrangement the name "Galilean". This type of eyepiece is still used in very cheap telescopes, binoculars and in opera glasses.
=== Convex lens ===