itself. For example, a Plössl with 45° apparent field of view in a 1.25 inch barrel would yield a maximum focal length of 35mm. 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 in. (24.5 mm) – This is the smallest standard barrel diameter and is usually found in toy store and shopping mall retail 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 in. (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 barrel itself 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 to take 30 mm filters. 2 in. (50.8 mm) – The larger barrel size in 2" eyepieces helps alleviate the limit on focal lengths. 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 to take 48 mm filters (or rarely 49 mm). 2.7 in. (68.58 mm) – 2.7" eyepieces are made by a few manufacturers. They allow for slightly larger fields of view. Many high-end focusers now accept these eyepieces. 3 in. (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