The Speed Booster Effective Aperture Calculator is a web utility that instantly calculates effective aperture, light transmission gain (in stops), and equivalent focal length for focal reducers without requiring any manual math. It is completely free to use.
Speed booster effective aperture calculator formulas
The ratio is the number printed on the booster, such as 0.71× or 0.64×. Every result above comes from four multiplications and one logarithm:
- Effective aperture = original f-number × ratio
- Effective focal length = original focal length × ratio
- Full-frame equivalent focal length = effective focal length × camera crop factor
- Full-frame equivalent aperture (for depth of field) = effective aperture × camera crop factor
- Light change in stops = −2 × log2 (ratio)
- Exposure follows the effective aperture. Depth of field follows the full-frame equivalent aperture.
Working example: 50mm f/1.8 on APS-C with a 0.71× speed booster
- Effective focal length: 50mm × 0.71 = 35.5mm
- Effective aperture: f/1.8 × 0.71 = f/1.28
- Full-frame equivalent focal length (1.5× crop): 35.5mm × 1.5 = 53.25mm
- Full-frame equivalent aperture: f/1.28 × 1.5 = f/1.92
- Light change: −2 × log2 (0.71) = +0.99 stops
Without the booster, the same lens on the same camera is 75mm f/1.8, or f/2.7 in full-frame terms. The booster gives you a wider view and a faster lens.
