Every holiday season, a box shows up with a little telescope and a big number on the front: 600x! 675x! The number is technically achievable. Put a tiny enough eyepiece in almost any telescope and you can hit it. What you'll see at that power is a dim, mushy, shaking blob that drifts out of view before you can focus.
Magnification is the least important number on a telescope, and also the most misunderstood. The good news is that the math is simple, and once you know it you can pick eyepieces with confidence and never get fooled by a box again. Let's do the math, then look at a real table for two scopes we sell.
The Short Answer
Telescope magnification equals the telescope's focal length divided by the eyepiece's focal length, and the maximum useful magnification is about 50x per inch (2x per millimeter) of aperture, often less because of the atmosphere.
- Example: a 1200 mm scope with a 25 mm eyepiece gives 48x. Swap in a 12 mm and you get 100x.
- Exit pupil (aperture ÷ magnification) tells you how bright the view is. About 2 to 5 mm is the comfortable range for most viewing.
- "600x" on a 60 to 70 mm scope is several times past its useful limit. Aperture, not magnification, decides what you can see.
- A Barlow lens multiplies every eyepiece's power, usually by 2x.
1. The Magnification Formula (It's One Division)
A telescope's main lens or mirror forms an image. The eyepiece is a magnifying glass you use to look at that image. Magnification depends on both.
Magnification = telescope focal length ÷ eyepiece focal length
Both numbers are printed on the gear. The Sky-Watcher Heritage 130 Tabletop Dobsonian has a 650 mm focal length (Sky-Watcher). With a 25 mm eyepiece, 650 ÷ 25 = 26x. With a 7 mm eyepiece, 650 ÷ 7 = 93x. Celestron lists exactly this kind of pairing for the Celestron StarSense Explorer DX 130AZ, which shares the 650 mm focal length: 26x with its 25 mm eyepiece and 65x with its 10 mm (Celestron specs).
Shorter eyepiece = more power
This trips up everyone once. The smaller the eyepiece number, the higher the magnification. A 25 mm is low power; a 5 mm is high power. So the same eyepiece gives different power in different scopes. A 12 mm gives 54x in a 650 mm scope and 100x in the 1200 mm Sky-Watcher Flextube 200P 8-inch GoTo Dobsonian.
Barlow lenses
A Barlow goes between the focuser and the eyepiece and multiplies its power. A 2x Barlow turns that 25 mm eyepiece into the equivalent of a 12.5 mm. Two eyepieces plus a Barlow gives you four magnifications, which is why it's often the smartest second accessory. A 8-24 mm zoom eyepiece is the other shortcut: one eyepiece that covers a whole range.
2. Maximum Useful Magnification, Exit Pupil and Why 600x Is a Lie
There is a ceiling, and it's set by aperture (the diameter of the main lens or mirror) and the air above you.
The 50x-per-inch rule
The classic guideline is about 50x per inch of aperture, or 2x per millimeter. Past that, you're magnifying blur the optics can't resolve. Sky & Telescope ties it to the exit pupil: a 0.5 mm exit pupil is the minimum useful size for any telescope, which works out to 50x per inch (Sky & Telescope: A Pupil Primer). For a 130 mm scope, that's about 260x. For an 8-inch (203 mm), about 400x. Manufacturers sometimes quote a bit higher. Celestron lists 307x for its 130 mm StarSense Explorer DX.
The atmosphere usually says no first
That ceiling assumes perfectly steady air, which almost never happens. Sky & Telescope's magnification myths article points out that bigger scopes, more affected by seeing, may top out at 20x or 30x per inch on most nights. In practice, most planetary viewing happens between about 100x and 200x. On a hot, hazy Georgia summer night, sometimes less.
So why is 600x a lie?
A typical department-store scope has a 60 or 70 mm lens. By the 2x-per-mm rule, it maxes out around 120x to 140x. At 600x on a 70 mm scope, the exit pupil is 70 ÷ 600 = 0.12 mm, about a quarter of the useful minimum. The image is huge, dim and soft. The number on the box is honest arithmetic and useless astronomy. Our beginner telescope guide explains what to look for instead: aperture and a steady mount.
Exit pupil: the brightness number
Exit pupil is the width of the beam of light leaving the eyepiece. It equals aperture ÷ magnification, or eyepiece focal length ÷ focal ratio. Sky & Telescope says most observers find views most satisfying with an exit pupil from 2 to 5 mm. Around 5 to 7 mm gives the brightest views of faint galaxies and nebulae (the eye's pupil opens to about 7 mm at most, less as we age). Around 1 mm and below is planet and double-star territory.
3. Worked Example: An Eyepiece Set for Two Real Telescopes
Here's what each of our Celestron X-Cel LX focal lengths does in a 130 mm f/5 tabletop Dobsonian and an 8-inch f/5.9 Dobsonian.
The table
| Eyepiece | Heritage 130 (650 mm, 130 mm) | Exit pupil | Flextube 200P (1200 mm, 203 mm) | Exit pupil | Good for |
|---|---|---|---|---|---|
| 25 mm | 26x | 5.0 mm | 48x | 4.2 mm | Finding objects, star clusters, galaxies |
| 18 mm | 36x | 3.6 mm | 67x | 3.0 mm | Nebulae, the whole Moon |
| 12 mm | 54x | 2.4 mm | 100x | 2.0 mm | Globular clusters, lunar detail |
| 9 mm | 72x | 1.8 mm | 133x | 1.5 mm | Jupiter, Saturn on average nights |
| 7 mm | 93x | 1.4 mm | 171x | 1.2 mm | Planets on steady nights |
| 5 mm | 130x | 1.0 mm | 240x | 0.85 mm | Planets and double stars in excellent seeing |
| 2.3 mm | 283x | 0.46 mm | 522x | 0.39 mm | Past the ~2x-per-mm limit on both scopes |
| Max useful (~2x per mm) | ~260x | 0.5 mm | ~400x | 0.5 mm | Rarely usable; the air sets the real limit |
Magnification = focal length ÷ eyepiece. Exit pupil = aperture ÷ magnification. Numbers are rounded.
How to build a set
- Low power (exit pupil 4 to 5 mm): your finding eyepiece. A 25 mm is the usual starting point.
- Medium power (about 2 to 2.5 mm): a 12 mm is the eyepiece most observers use most.
- High power (about 1 to 1.5 mm): a 7 mm in a Heritage 130 or a 9 mm in an 8-inch, for the Moon and planets.
- Fill gaps with a 2x Barlow instead of more eyepieces. Pick focal lengths that don't land on top of each other once Barlowed.
Field of view matters too
Apparent field (40°, 60°, 82° and so on) is how wide the view looks through the eyepiece. True field is roughly apparent field ÷ magnification. A 60° eyepiece at 100x shows about 0.6°, a bit more than the full Moon. Wider eyepieces make high power easier on a Dobsonian, because objects take longer to drift out of view.
Check the barrel size
Most beginner scopes take 1.25-inch eyepieces. Larger Dobsonians often have a 2-inch focuser with a 1.25-inch adapter (the Flextube 200P does, per Sky-Watcher), which opens up long, wide-field 2-inch eyepieces.
Conclusion
Forget the number on the box. Divide focal lengths to get magnification, stay under about 2x per millimeter of aperture, and expect the air to cap you lower on most nights. Then build a small set of low, medium and high power, add a Barlow, and you'll have the right view for everything from the Pleiades to Saturn. If your stars won't come to a sharp point at high power, your reflector may need a quick collimation.
Browse our telescope eyepieces and Barlow lenses, or stop by the store and we'll help you work out the exact powers for your scope.
Gear Mentioned in This Guide
Frequently Asked Questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length. A 1200 mm telescope with a 25 mm eyepiece gives 48x.
What is the maximum useful magnification of a telescope?
About 50x per inch of aperture, or 2x per millimeter: roughly 260x for a 130 mm scope and 400x for an 8-inch. Atmospheric turbulence usually limits you to less.
Is 600x magnification good for a telescope?
Not on a small scope. A 60 to 70 mm telescope tops out around 120x to 140x. At 600x the image is dim, blurry and shaky.
What is exit pupil?
The width of the light beam leaving the eyepiece, equal to aperture divided by magnification. Larger exit pupils give brighter views. About 2 to 5 mm is comfortable for most viewing.
What does a Barlow lens do?
It multiplies the magnification of any eyepiece, usually by 2x, so a 25 mm eyepiece behaves like a 12.5 mm.
What eyepiece is best for viewing planets?
One that gives roughly 100x to 200x in your scope, for example a 7 mm in a 650 mm focal length telescope (93x) or a 9 mm in a 1200 mm telescope (133x).
