Telescope magnification calculator
This calculator tells you how much a given telescope-and-eyepiece pairing magnifies, and whether that magnification is sensible for your scope’s aperture. It is useful when buying eyepieces, planning an observing session, or working out why a high-power view looks disappointing.
When to use this tool
- Comparing eyepiece options before you buy
- Checking whether a Barlow lens is worth adding
- Understanding why a high-power view looks dim or soft
- Planning which eyepiece to start with for a specific target (planets, clusters, wide nebulae)
How it works
Three quantities come from two simple ratios, all in millimetres:
Magnification = telescope focal length ÷ eyepiece focal length
Focal ratio (f/number) = telescope focal length ÷ aperture
Maximum useful magnification ≈ 2 × aperture (mm)Magnification grows as you fit a shorter eyepiece. The maximum-useful limit exists because beyond about 2× the aperture in mm you spread the collected light too thin and exceed the optics’ resolving power, so the image dims and softens. The tool flags magnifications above that limit.
Example
A 1000 mm focal-length telescope with a 25 mm eyepiece gives 1000 ÷ 25 = 40×. With a 200 mm aperture the focal ratio is 1000 ÷ 200 = f/5, and the maximum useful magnification is 2 × 200 = 400×. So 40× is comfortable, while a 4 mm eyepiece (250×) is still within the limit but a 2 mm eyepiece (500×) would over-magnify.
| Eyepiece (1000 mm scope) | Magnification |
|---|---|
| 32 mm | 31× |
| 25 mm | 40× |
| 10 mm | 100× |
| 5 mm | 200× |
Everything runs in your browser — no uploads.
Choosing the right eyepiece for different targets
The useful magnification range for a telescope varies by what you are looking at:
- Wide nebulae and open clusters benefit from low power (20–60×) to fit the whole object in the field of view with bright background contrast.
- Globular clusters improve with moderate power (80–150×) to begin resolving individual stars.
- Planets usually show the most detail between 150–250×, up to the seeing limit and your scope’s maximum useful magnification.
- Double stars can push to or near the Dawes limit — the magnification needed depends on the pair’s separation, not just your aperture.
Minimum useful magnification
Just as there is a maximum, there is also a practical minimum. Very low
magnification makes the exit pupil — the beam of light leaving the eyepiece —
wider than about 7 mm, which is the maximum the dark-adapted eye can use.
Magnification below aperture (mm) / 7 wastes collected light by sending it
outside the pupil.
Barlow lenses
A Barlow multiplies the effective focal length, so a 2× Barlow on a 25 mm eyepiece behaves like a 12.5 mm eyepiece. This lets you reach high power with eyepieces that have a larger, more comfortable eye lens. The trade-off is two extra glass surfaces, which can reduce contrast slightly; high-quality Barlows minimise this.