How to use the Focal Ratio Calculator
You need two pieces of information to calculate your telescope's focal ratio:
- The focal length of the telescope in millimetres.
- The aperture (the diameter of the main lens or mirror) in millimetres.
Press Calculate and you will get the focal ratio as an f-number.
What is the focal ratio?
The focal ratio formula for a telescope is:
Focal Length (mm) / Aperture (mm) = Focal Ratio (f-number)
For example, a telescope with a 1000mm focal length and a 200mm aperture has a focal ratio of f/5. A lower focal ratio (a "faster" scope) gives a wider field and shorter exposures, which is useful for deep-sky astrophotography. A higher focal ratio (a "slower" scope) gives more magnification, which suits planetary and lunar viewing.