Telescope Magnification Calculator
Magnification and true field of view from your telescope's focal length and any eyepiece.
- Magnification (×)36
- True field of view (°)1.39
- Full Moons that fit across the view2.7
Inputs used
- Telescope focal length900 mm
- Eyepiece focal length25 mm
- Eyepiece apparent field of view50 °
About this calculator
Magnification is simply the telescope's focal length divided by the eyepiece's: a 900 mm telescope with a 25 mm eyepiece gives 36×, and swapping in a 10 mm eyepiece raises that to 90×. Shorter eyepiece, higher power.
Higher isn't automatically better. Divide the eyepiece's apparent field of view by the magnification to get the true field — the patch of sky you actually see. At 36× with a 50° eyepiece you get 1.4°, nearly three full Moons wide and great for star clusters; at 180× you're down to 0.28°, a tight crop suited to planets and double stars.
A practical ceiling: about 2× per millimetre of aperture (a 130 mm telescope tops out near 260×). Beyond that, the image grows dimmer and blurrier without revealing anything new — the atmosphere usually caps useful power at 150–250× anyway.
Quick reference
Magnification by eyepiece and telescope focal length
Common eyepiece sizes against popular telescope focal lengths (mm).
| Eyepiece focal length | 650 mm | 900 mm | 1,200 mm | 2,000 mm |
|---|---|---|---|---|
| 4 mm | 163 | 225 | 300 | 500 |
| 6 mm | 108 | 150 | 200 | 333 |
| 10 mm | 65 | 90 | 120 | 200 |
| 15 mm | 43 | 60 | 80 | 133 |
| 25 mm | 26 | 36 | 48 | 80 |
| 32 mm | 20 | 28 | 38 | 63 |
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length, both in millimetres. A 1200 mm telescope with a 10 mm eyepiece gives 1200 ÷ 10 = 120×.
What magnification do I need to see Saturn's rings?
The rings become visible around 25–30× and look clearly ring-shaped at 100× or more, seeing conditions permitting. Steady air matters more than raw power — a crisp 120× beats a wobbly 250× every night.
Why does everything look dimmer at high magnification?
The telescope gathers a fixed amount of light, and magnifying spreads it over a larger apparent area. Doubling the magnification makes the view four times dimmer per unit of sky — one reason faint galaxies are hunted at low power.
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Results are informational only — not financial, medical or legal advice.