Telescope calculator
Magnification, exit pupil & resolution of your telescope.
Before buying a telescope or a new eyepiece, the numbers are worth a look. Enter your telescope’s aperture and focal length plus the eyepiece focal length — the calculator shows magnification, sensible maximum magnification, exit pupil and resolving power. That saves you from costly mistakes.
Enter your telescope & eyepiece
Magnification
120×
Max useful
300×
Exit pupil
1.3 mm
Focal ratio
f/8
Resolution (Dawes)
0.77″
Magnification = focal length ÷ eyepiece. Max useful ≈ 2× aperture (mm). Dawes limit = 116″ ÷ aperture.
The key figures
Magnification = telescope focal length ÷ eyepiece focal length. The sensible maximum magnification is roughly 2× the aperture in millimetres. The exit pupil (aperture ÷ magnification) should not exceed about 6–7 mm at night.
Why "more magnification" is a myth
Cheap telescopes advertise "525×" — but beyond the physical limit the image only turns dim and mushy. What matters is aperture: it determines how much light and detail are possible at all.
Frequently asked questions
What magnification makes sense?
As a rule of thumb, 1–1.5× the aperture in mm for sharp images, at most 2×. A 150 mm telescope thus about 150–300×.
What is the exit pupil?
The cone of light entering your eye. If it is too large (over ~7 mm), light is wasted around the pupil — especially with age.
Aperture or focal length — which matters more?
Aperture. It gathers light and sets the resolution. Focal length and eyepiece only determine magnification.
What does resolving power mean?
The smallest angle the telescope can still separate (Dawes limit). The larger the aperture, the finer the detail — e.g. close double stars.
Do I need many eyepieces to start?
No. Two or three well-chosen focal lengths plus a Barlow lens cover most observing.
Which telescope for beginners?
A Dobsonian with 150–200 mm aperture offers the best ratio of aperture, simplicity and price. More in our recommendations.
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