Telescope Magnification & F-Ratio Calculator
Find your telescope's magnification, focal ratio (f/#) and exit pupil from the scope focal length, eyepiece focal length and aperture.
mm
mm
mm
Times larger than the naked eye
- 1
Scope focal length
F = 1,200 mm = 1,200 - 2
Magnification = scope ÷ eyepiece focal length
1,200 ÷ 25 = 48
How does this calculator work?
Telescope magnification equals the telescope focal length divided by the eyepiece focal length. The focal ratio (f/#) is the focal length divided by the aperture, and the exit pupil is the aperture divided by the magnification. A 1200 mm scope with a 25 mm eyepiece and 200 mm aperture gives 48×, f/6 and a 4.2 mm exit pupil.
Formula
How this is calculated
Magnification tells you how many times larger an object appears compared with the naked eye. It is simply the telescope focal length divided by the eyepiece focal length, so a 1200 mm scope with a 25 mm eyepiece gives 48×. Swapping in a shorter eyepiece increases magnification. Both focal lengths must be in the same units (millimetres here).
The focal ratio, written f/#, is the telescope focal length divided by the aperture (the diameter of the main lens or mirror). A lower f-ratio is "faster" — it produces a brighter, wider image well suited to deep-sky viewing, while a higher f-ratio gives more contrast for the Moon and planets. The exit pupil is the width of the beam of light leaving the eyepiece, equal to the aperture divided by the magnification (equivalently, the eyepiece focal length divided by the f-ratio).
All three inputs must be positive, and the eyepiece focal length and aperture cannot be zero. As a practical guide the useful maximum magnification is roughly 2× the aperture in millimetres, and an exit pupil larger than about 7 mm wastes light because it exceeds the dilated human pupil.
Frequently asked questions
Most observing is done between 50× and 150×. The maximum useful magnification is about 2× the aperture in millimetres (e.g. roughly 400× for a 200 mm scope) before the image becomes dim and blurry.
It is the focal length divided by the aperture. A low number like f/5 is "fast" and gives bright, wide-field views, while a high number like f/10 gives narrower, higher-contrast views ideal for planets.
The exit pupil is how wide the light beam is as it enters your eye. If it is much larger than your eye's pupil (about 5-7 mm in the dark) some light is wasted, while a very small exit pupil makes the image dim.
Also known as
TG we-Calculate Editorial Team. (2026). Telescope Magnification & F-Ratio Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/telescope-magnification-calculator
TG we-Calculate Editorial Team. "Telescope Magnification & F-Ratio Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/telescope-magnification-calculator.
TG we-Calculate Editorial Team, "Telescope Magnification & F-Ratio Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/telescope-magnification-calculator
@misc{wecalculate_telescope_magnification_calculator, title = {Telescope Magnification & F-Ratio Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/telescope-magnification-calculator}}, year = {2026}, note = {TG we-Calculate} }
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