Choosing the Right Eye Pieces

The Greater Hazleton Area Astronomical Society

The optical elements of Eyepieces allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. It may seem like a small link in the chain, but it has a large effect on your telescope’s optical system, and finding suitable eyepieces will greatly enhance its potential. 

With so many options to choose from, selecting the right set of eyepieces for you and your telescope can seem a little tricky. This guide offers some insight and explanations on different eyepiece types, specifications, and how it all ties together to optimize your astronomy and astrophotography sessions!


Focal Length is an important specification to consider when determining the magnification, also known as power, of the combination of an eyepiece and the telescope it is being used with. The following formula will help you determine the magnification based on your eyepiece and telescope’s specifications:Magnification  = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)
For example:

  • A 20 mm eyepiece on a 2000 mm telescope (2000/20) gives you 100 power (100x). This makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye. 

Note: When using your telescope at different powers, you generally have a choice of a small, sharp, and bright image at lower magnification; or a larger, yet blurred and dim image at higher magnification. The reason being, that the telescope gathers a fixed amount of light, and at higher magnifications, the same amount of light is being spread over a larger area, resulting in a dimmer image.


An eyepiece’s Apparent Field of View (AFOV) is expressed in degrees (°). It is how much of the sky is seen edge-to-edge through the eyepiece alone. AFOV’s range from narrow (25° – 30°) to an extra-wide angle (80° or more).

An eyepiece’s true field of view is the angle of sky seen through the eyepiece when it’s attached to the telescope. The true field can be calculated using the following formula:

True Field = Apparent Field / Magnification

For example, suppose you have an 8-inch Schmidt-Cassegrain telescope with a 2000 mm focal length, and a 20 mm eyepiece with a 50° apparent field. The magnification would be 2000 mm / 20 mm = 100x. The true field would be 50\100, or 0.5° – about the same apparent diameter as the full moon.


Eye Relief refers to the distance between your eye and the eyepiece lens when the image is in focus. Eye relief is traditionally in proportion with focal length: The shorter the focal length, the shorter the eye relief. However, some of the more modern eyepiece designs provide long-eye relief regardless of focal length, which is especially beneficial to those who wear glasses. If you like to keep your glasses on while using a telescope, the eye relief of an eyepiece is an important specification to consider (we recommend looking at long-eye relief eyepieces). 

2 mm – 4.9 mm Eyepieces

These produce very high magnifications and work best on long focal length refractors and Schmidt-Cassegrains. Unless you have very steady seeing conditions, this range more than likely will produce too much magnification for other telescope types.

5 mm – 6.9 mm Eyepieces

These make good planetary detail and double star eyepieces for long focal length telescopes and will work satisfactorily in shorter focal length telescopes with steady seeing conditions.

7 mm – 9.9 mm Eyepieces

Ideal high magnification eyepieces for shorter focal length telescopes, and serve as good planetary, double star and lunar detail units.

10 mm – 13.9 mm Eyepieces

Good to use across all focal lengths and offer great background darkening capabilities for studying planetary nebula, small galaxies, planetary details and lunar details.

14 mm – 17.9 mm Eyepieces

A great mid-range magnification for all focal lengths and helps resolve globular clusters, galaxy details, and spot planetary nebulae.

18 mm – 24.9 mm Eyepieces

Works nicely on long focal length telescopes to show wide field and extended objects. Shorter focal length telescopes will enjoy great mid-range magnification of galaxy clusters and large open clusters.

31 mm – 39.9 mm Eyepieces

Longer focal lengths are good for large nebula and open clusters. Shorter focal lengths, are great for large objects such as the Orion nebula, views of the full lunar disc, large open clusters, and more. It also makes for good “locator” eyepieces in all focal lengths.

31 mm – 39.9 mm Eyepieces

These are well suited for shorter focal length telescopes for extended views and large, starry fields.

40 mm Eyepieces

These are exclusively the domain of shorter focal length telescopes. This magnification range is superb for showing large, starry vistas as well as extended nebulae with star fields and etc.

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