Telescope Magnification & FOV Calculator
Find a telescope-and-eyepiece combination’s magnification, true field of view and exit pupil — and check it against the scope’s maximum useful power.
How it works
Magnification = scope focal length ÷ eyepiece focal length · True FOV = apparent FOV ÷ magnification · Exit pupil = aperture ÷ magnification · Max useful ≈ 2 × aperture(mm)
Magnification is set by the pair of focal lengths, so a shorter eyepiece zooms in further; the true field of view then shrinks in proportion, which is why high power shows a small patch of sky. Exit pupil — the width of the beam leaving the eyepiece — should stay below about 7 mm (your dark-adapted pupil) or light is wasted. Every scope also has a maximum useful magnification of roughly twice its aperture in millimetres: push past it and you enlarge blur rather than detail, giving a dim, mushy image.
Worked example
A 150 mm f/8 scope has a 1,200 mm focal length. With a 25 mm, 52° eyepiece the magnification is 1200 ÷ 25 = 48×; true field is 52 ÷ 48 = 1.08°; exit pupil is 150 ÷ 48 = 3.1 mm. The scope’s useful ceiling is about 2 × 150 = 300×, so 48× is a comfortable low-power view.
Frequently asked questions
Why can’t I just use the highest magnification possible?
Beyond about twice the aperture in millimetres, you magnify the blur from the atmosphere and the optics themselves rather than revealing new detail. The image also dims because the same light is spread over a larger apparent area. On most nights the atmosphere caps useful power well below the theoretical maximum.
What is exit pupil and why does it matter?
Exit pupil is the diameter of the light beam leaving the eyepiece, equal to aperture ÷ magnification. If it exceeds your eye’s dark-adapted pupil (about 7 mm), some collected light misses your eye. Very small exit pupils (below ~0.5 mm) make the view dim and reveal floaters in your eye.
How do I find the eyepiece’s apparent field of view?
It is a spec of the eyepiece, printed on it or in its manual — typically 50–52° for a Plössl and 60–82° for wide-angle designs. Dividing it by the magnification gives the true field, the amount of actual sky you see.
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This calculator is for educational purposes. Double-check important results before acting on them.