Magnification & True-FOV Calculator
Magnification isn't a property of the telescope — it's the scope and the eyepiece working together. Swap the eyepiece and you change the magnification, the field of view and the brightness of the image. Enter your scope's focal length and an eyepiece below to see exactly what that combination gives you — and whether you're matching it well or pushing past what the scope can usefully do.
On the tube or in the manual — e.g. an 8″ f/5.9 Dob is 1200 mm.
Printed on the eyepiece — common kit values are 25 mm and 10 mm.
From the eyepiece spec. Plössl ≈ 52°; wide-field 68–82°. Default 52°.
A Barlow multiplies magnification by sitting before the eyepiece.
Add it to get exit pupil and an over-magnification check (max useful ≈ 2× aperture). Leave blank to skip.
How this is calculated
- Magnification =
telescope focal length ÷ (eyepiece focal length ÷ Barlow). A Barlow effectively shortens the eyepiece, so it multiplies the magnification. - True field of view =
apparent field of view ÷ magnification— the actual patch of sky you see, in degrees. (The full Moon is about 0.5° across, a handy yardstick.) - Exit pupil =
aperture ÷ magnification(mm) — the width of the light beam leaving the eyepiece. Below ~0.5 mm the view dims and "floaters" appear; above ~7 mm an adult eye can't take in all the light, so some is wasted. - Maximum useful magnification ≈
2 × aperture in mm. Past it you get "empty magnification" — a bigger but softer image, not more detail. The practical lowest useful power is roughly where the exit pupil hits ~7 mm.
These are physics-derived from the numbers you enter — not test results. The atmosphere ("seeing"), optical quality and your target all decide how much magnification is actually worth using on the night.
Want to fill the gaps in your magnification range? A good beginner set is a low-power wide-field eyepiece, a mid-power, and a Barlow to double them — compared on focal length, field of view and build in our best telescope eyepieces guide. Not sure your scope can reach what you want to see? Start with the What Can I See aperture calculator.
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