Free Focal Reducer Effect Calculator
Calculate the effective focal length and f-ratio after adding a focal reducer: f_eff = f * reduction_factor, N_eff = N * reduction_factor.
mm
Effective Focal Length
630.00 mm
Effective f-Ratio6.30
Speed Gain Factor2.52 x
Effective Focal Length vs Native Focal Length
Focal Reducer Effect
A focal reducer shortens the effective focal length, widening the FOV and making the system faster.
Formulas
f_eff = f * R N_eff = N * R
where R is the reduction factor (e.g., 0.63).
The speed gain (for extended objects) scales as 1/R squared, because the same light is concentrated into a smaller image.
Example Calculation
1000 mm f/10 scope with a 0.63x reducer.
- 01f_eff = 1000 * 0.63 = 630 mm
- 02N_eff = 10 * 0.63 = f/6.3
- 03Speed gain = 1 / 0.63^2 = 2.52x faster