Free Telescope Focal Ratio Calculator
Calculate the focal ratio (f-number) of a telescope: f/# = focal_length / aperture.
mm
mm
Focal Ratio
6.0 f/
Focal Ratio vs Focal Length
Focal Ratio (f-Number)
f/# = Focal Length / Aperture
Lower f-numbers ("faster" scopes) collect light over a wider cone and are better for astrophotography of faint objects. Higher f-numbers ("slower") give longer focal lengths for planetary detail.
Example Calculation
1200 mm focal length, 200 mm aperture.
- 01f/# = 1200 / 200 = f/6
- 02This is a moderately fast scope, good for both visual and imaging.