The short answer
Choose eyepieces by the views they give in your telescope, not by their numbers alone. Three formulas do most of the work: magnification is the telescope’s focal length divided by the eyepiece’s; exit pupil is the aperture divided by the magnification; and true field of view is roughly the eyepiece’s apparent field divided by the magnification. Most beginners are well served by three eyepieces: a low-power one for finding things and large objects, a medium one for most targets and a high-power one for the Moon, planets and double stars.
Focal length and magnification
Every eyepiece is labeled with its focal length in millimeters. The shorter the focal length, the higher the magnification it gives:
Magnification = telescope focal length ÷ eyepiece focal length
In a telescope with a 1,200 mm focal length, a 25 mm eyepiece gives 48× and a 10 mm gives 120×. The same 10 mm eyepiece in a 650 mm telescope gives only 65×. That’s why an eyepiece’s focal length means nothing on its own; you have to know the telescope it goes in.
There’s an upper limit to useful magnification of roughly 2× per millimeter of aperture, and the atmosphere usually caps planetary views at about 200–250×. See the best magnification for planets.
Exit pupil
The exit pupil is the width of the beam of light leaving the eyepiece and entering your eye. It decides how bright extended objects look.
Exit pupil = aperture ÷ magnification, or equivalently, eyepiece focal length ÷ telescope focal ratio.
| Exit pupil | Power | Good for |
|---|---|---|
| About 4–7 mm | Low | Finding objects, large clusters, wide nebulae, sweeping the Milky Way |
| About 2–3 mm | Medium | Most galaxies, smaller clusters, general use |
| About 1–1.5 mm | High | The Moon, planets, globular clusters, planetary nebulae |
| About 0.5–1 mm | Very high | Planets and close double stars on steady nights only |
Your eye’s pupil sets the upper end. It opens to about 7 mm in the dark when you’re young and closes to 5 mm or so as you age. A larger exit pupil than your own pupil wastes light. Below about 0.5 mm, images get dim and the floaters in your eye become distracting.
Apparent and true field
Eyepieces are also described by their apparent field of view, the angle of the circle of view you see when you look in. Simple designs show around 40–50 degrees; wide-angle designs show 60–70 degrees, and some show 80 degrees or more. A wider apparent field feels more immersive, like a bigger window.
The true field is how much of the sky that window actually shows:
True field ≈ apparent field ÷ magnification
A 50-degree eyepiece at 50× shows about 1 degree of sky, roughly two full Moons side by side. An 82-degree eyepiece at the same 50× shows about 1.6 degrees. A wider true field makes it easier to find objects and to fit more of large ones, such as the Pleiades, into the view. The magnification calculator works all three numbers out for your setup.
Building a simple set
Here’s an example for a 200 mm (8-inch) Dobsonian with a 1,200 mm focal length, a focal ratio of f/6. Work out your own with the calculator; the principle is the same for any telescope.
| Eyepiece | Magnification | Exit pupil | Role |
|---|---|---|---|
| 25 mm | 48× | About 4.2 mm | Finding, clusters, large nebulae |
| 12 mm | 100× | 2 mm | Galaxies, general use, the whole Moon |
| 6–7 mm | About 170–200× | About 1–1.2 mm | Planets, Moon detail, double stars |
Spacing the powers by a factor of about 1.5 to 2 avoids buying eyepieces that duplicate each other. A good-quality Barlow lens can double the set, but check that the doubled powers don’t simply repeat ones you already have.
Other things that matter
- Barrel size. Most eyepieces fit 1.25-inch focusers. Many larger telescopes also take 2-inch eyepieces, which allow the widest true fields at low power.
- Eye relief. How far your eye sits from the lens when you see the whole field. If you observe with glasses, look for longer eye relief; many observers find around 15 mm or more comfortable. Short-focal-length eyepieces of simple design can have very little.
- Your telescope’s focal ratio. Fast telescopes (low f-numbers, such as f/4 or f/5) are less forgiving and show blurry edges with cheaper wide-angle eyepieces. Slow ones (f/10 and above) work well with simple designs.
- Zoom eyepieces. A decent zoom covers a range of powers in one eyepiece and is convenient, usually with a narrower field at the low end.
If you’re still choosing a telescope, start with binoculars or telescope and what you can see with a small telescope.