Focal Ratio (f-number) Calculator
The focal ratio tells you how “fast” a telescope is — key for imaging and for how bright, wide the views are. Enter the aperture and focal length to get the f-number.
Focal Ratio (f-number) Calculator
Why use the Focal Ratio (f-number) Calculator
“Fast” and “slow” scopes suit different targets and imaging. This tells you which one you have.
How to get your numbers
- Aperture in mm.
- Focal length in mm.
Recommended for this job
How to use it
- Enter the aperture (mirror or lens diameter) in mm.
- Enter the focal length in mm.
- Compare the result: below f/6 is fast, above f/10 is slow.
Frequently asked questions
How do I calculate focal ratio?
Divide the focal length by the aperture. A 750 mm focal length with a 150 mm aperture is f/5.
What is a “fast” telescope?
A low focal ratio (say f/4–f/5) gives brighter, wider fields — ideal for deep-sky viewing and astrophotography with shorter exposures.
Is a “slow” telescope bad?
No — a high focal ratio (f/10+) gives high-contrast, high-magnification views that shine on the Moon and planets.
What focal ratio is best for astrophotography?
Faster is generally better: f/4–f/6 gathers light quickly for shorter exposures on faint deep-sky objects.
What focal ratio is best for planets?
Slower scopes (f/10 and up) provide the long focal length and high magnification that suit the Moon, planets and double stars.
Does focal ratio affect brightness when viewing by eye?
For extended objects, yes — at the same magnification, a faster scope of equal aperture shows a wider, brighter field via a larger exit pupil.
