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Choosing Telescope Eyepieces

Eyepieces for telescopes come in many different designs to fit many different budgets. Here is an explanation of B&H´s terminologies as well as some tips on how to select them:

Long Eye Relief

The distance from the top of the eyepiece at which your eye can be while still being able to see the entire field of view afforded by an optical device is known as the eye relief. When an eyepiece has minimal eye relief, such as 10mm or less, the entire view will not be possible to see for those wearing eyeglasses because their glasses extend more than 10mm beyond the front of their eyes.

Additionally, short eye relief eyepieces can be tiring to observe with for those who don't wear glasses. It is very common for short focal length eyepieces, such as those of 10mm or less, to have minimal eye relief. Eyepieces with eye relief of 15mm or more are classified as "long eye relief" here, and 15mm is usually sufficient for most users.

Wide Apparent Field

An eyepiece can show a narrow slice of sky while seeming to have a view that surrounds your vision, just as easily as an eyepiece can show a wide chunk of sky while seeming to have a view that's narrow and further away in the eyepiece.

This is because of "apparent field of view", which is a way to express the concept of how wide the view feels within the eyepiece itself. Nearly any observer enjoys a wide apparent view because it makes you feel like you’re 'swimming' in space, but this does not come without disadvantages. Generally speaking, those eyepieces which have a wide apparent field will usually have less eye relief than an eyepiece which has a narrower apparent field. Additionally, telescopes which have a 'fast' aperture can expose an eyepiece's optical flaws, while the same eyepiece used on a 'slower' telescope can appear brilliantly sharp and free of distortions. If you have a telescope with an aperture of around f/5.0 and wish to use an eyepiece with a wide apparent field, you'll need to spend quite a bit of money to get a view which is enjoyable from edge to edge.

Wide Field/Long Relief

This designation is used to highlight eyepieces which achieve BOTH specialties of longer eye relief and a wider apparent field in one unit. These eyepieces are often large, heavy and expensive, but they will provide the best viewing experience for eyeglass users and those without eyeglasses.

Standard

This designation refers to eyepieces which have an apparent field of view of 59 degrees or less and an eye relief of less than 15mm. The advantage of these types is that they are often small, less expensive, and less of an optical challenge for your telescope.
It is a good idea to space out the focal lengths of your eyepiece collection so that you can observe at different magnifications to match the conditions. Thus if your telescope came with just a 25mm eyepiece, it would make sense to buy another that was perhaps 30-40mm, one around 15mm or so, and one below 10mm. Within one brand and style of eyepiece, it is often true that each eyepiece in that collection will be parfocal with the others. This means that when you switch eyepieces while observing the same object, the telescope will require little or no refocusing with the new eyepiece. This means less vibration to the scope and more real time enjoying the view at the eyepiece. But the best advice of all is this; buy the best quality eyepieces that are within your budget. You'll use these every time you observe and they critically affect your viewing experience, so it is not similar to saving by buying a cheaper filter that gets used only occasionally.

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