The formula
Magnification equals telescope focal length divided by eyepiece focal length, both in millimeters.
Magnification = telescope focal length ÷ eyepiece focal length
A 1000mm telescope with a 25mm eyepiece gives 40x. Swap in a 10mm eyepiece and you get 100x. Your telescope’s focal length is usually printed on the tube or listed in the manual.
Chart 1: a 1000mm focal length telescope
This example assumes a 100mm aperture (f/10). Values are rounded.
| Eyepiece | Magnification | Exit pupil (approx.) |
|---|---|---|
| 40mm | 25x | 4.0 mm |
| 32mm | 31x | 3.2 mm |
| 25mm | 40x | 2.5 mm |
| 20mm | 50x | 2.0 mm |
| 15mm | 67x | 1.5 mm |
| 12mm | 83x | 1.2 mm |
| 10mm | 100x | 1.0 mm |
| 8mm | 125x | 0.8 mm |
| 6mm | 167x | 0.6 mm |
| 4mm | 250x | 0.4 mm |
Chart 2: an 8-inch Dobsonian with 1200mm focal length
This example assumes a 200mm aperture (about f/6). Values are rounded.
| Eyepiece | Magnification | Exit pupil (approx.) |
|---|---|---|
| 40mm | 30x | 6.7 mm |
| 25mm | 48x | 4.2 mm |
| 20mm | 60x | 3.3 mm |
| 15mm | 80x | 2.5 mm |
| 10mm | 120x | 1.7 mm |
| 8mm | 150x | 1.3 mm |
| 6mm | 200x | 1.0 mm |
| 4mm | 300x | 0.7 mm |
Same eyepiece, different telescope, different power. A chart is only valid for the telescope it was made for. If yours differs, just divide your focal length by each eyepiece.
Limits to keep in mind
Maximum useful magnification
A commonly quoted rule of thumb is about 50x per inch of aperture, or 2x per millimeter. That makes roughly 200x for a 100mm telescope and 400x for a 200mm one. Treat these as upper limits for ideal conditions, not goals. In practice the air, the optical quality and collimation often limit you to less.
Minimum useful magnification
Too little power can make the exit pupil larger than your eye’s pupil, which wastes light. Dividing the aperture in millimeters by about 7 gives a rough lowest power for a dark-adapted eye, although this varies with age and individual eyes.
Exit pupil
Exit pupil is the width of the beam of light leaving the eyepiece, in millimeters. Calculate it as aperture divided by magnification, or eyepiece focal length divided by the telescope’s focal ratio. Larger exit pupils give brighter, lower-power views; smaller ones give higher power and dimmer images. Exit pupils much below 0.5 mm usually look dim and soft.
Common questions
What is a decent magnification for a telescope? There is no single answer. Many observers spend much of their time between roughly 30x and 150x, going higher only when the air is steady. Start low and increase gradually.
What size eyepiece is best for viewing planets? Planets are small, so higher power helps. On many telescopes that means eyepieces of roughly 4mm to 10mm, or a Barlow lens, depending on focal ratio and conditions. Check the exit pupil and avoid pushing past the limits above.
Is a 10mm or 20mm eyepiece more powerful? The 10mm, by a factor of two on the same telescope.
What is a 40mm eyepiece good for? Low power and wide fields: finding objects, large star clusters, and big nebulae. On the 8-inch above it gives 30x, a good starting view. Note that 40mm eyepieces are often 2 inch barrel; see eyepiece barrel sizes before buying.
Using a Barlow lens
A 2x Barlow lens doubles the power of whichever eyepiece you use with it. A 20mm eyepiece with a 2x Barlow behaves roughly like a 10mm eyepiece. This is an inexpensive way to fill gaps in a small set, though a dedicated eyepiece may give a more comfortable view.
Practical tips
- Start with the lowest power, center the object, then step up.
- Be skeptical of maximum power claims on telescope boxes; they are often unrealistic.
- If the view looks blurry at high power, drop down one step and refocus.
See also the full telescope eyepieces guide.