This speed booster effective aperture calculator gives you the effective aperture (f-number × booster ratio) and full-frame equivalent focal length (focal length × ratio × crop factor) for any lens on a focal reducer. Enter your lens, booster ratio and camera crop factor below. Results update as you type and run in your browser.
Speed booster & effective aperture calculator
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Exposure follows the effective aperture. Depth of field follows the full-frame equivalent aperture.
Speed boosters are made for sensors smaller than the lens image circle. On a full-frame or larger sensor the shrunken image circle will not cover the frame, so expect vignetting.
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.
Worked 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.
Speed booster ratios and stops gained
Effective f-number for common ratios. These are theoretical values; glass absorption lowers the real gain slightly.
| Ratio | Light change | f/1.4 becomes | f/2 becomes | f/2.8 becomes | f/4 becomes |
|---|---|---|---|---|---|
| 0.71× | +0.99 stops | f/0.99 | f/1.42 | f/1.99 | f/2.84 |
| 0.64× | +1.29 stops | f/0.9 | f/1.28 | f/1.79 | f/2.56 |
| 0.58× | +1.57 stops | f/0.81 | f/1.16 | f/1.62 | f/2.32 |
| 1.4× teleconverter | −0.97 stops | f/1.96 | f/2.8 | f/3.92 | f/5.6 |
| 2.0× teleconverter | −2 stops | f/2.8 | f/4 | f/5.6 | f/8 |
Camera crop factors for the speed booster calculator
The last column is the combined crop factor (camera crop × 0.71). At about 1.07, a 0.71× booster on APS-C nearly restores a full-frame field of view.
| Sensor format | Crop factor | With a 0.71× booster |
|---|---|---|
| APS-C (Sony, Nikon, Fujifilm) | 1.5 | about 1.07 |
| APS-C (Canon) | 1.6 | about 1.14 |
| Micro Four Thirds | 2.0 | 1.42 |
| Full frame | 1.0 | Not applicable: a booster is not used on full frame |
How a speed booster changes your lens
A speed booster is a focal reducer: an adapter with glass elements that condenses the lens’s full image circle onto a smaller sensor. The sensor sees a wider field of view and the light is concentrated, which lowers the f-number by the ratio. It only makes sense for lenses whose image circle is larger than your sensor. For the full explanation, read the guide to speed booster effective aperture.
Speed booster effective aperture calculator FAQ
What is effective aperture with a speed booster?
Effective aperture is the lens f-number multiplied by the speed booster ratio. A 50mm f/1.8 lens with a 0.71× booster behaves like f/1.28 for exposure (1.8 × 0.71 = 1.278).
How do I calculate the full-frame equivalent focal length?
Multiply the lens focal length by the booster ratio, then by the camera crop factor. Example: 50mm × 0.71 × 1.5 = 53.25mm full-frame equivalent.
Can I use this calculator for teleconverters?
Yes. Choose 1.4× or 2.0×, or enter a custom ratio above 1. The same formulas apply, and the light change is negative because a teleconverter loses light.
Does the calculator account for light lost in the adapter glass?
No. It assumes ideal optics, so the results are theoretical f-stops. Adapter glass absorbs a little light, which is why cinema users compare T-stops. The guide covers the T-stop vs f-stop difference in detail.
What crop factor should I enter if my camera is not in the list?
Divide 43.27 by your sensor diagonal in millimetres. Common values are 1.5 for Sony, Nikon and Fujifilm APS-C, 1.6 for Canon APS-C and 2.0 for Micro Four Thirds.
Method and limits
This speed booster effective aperture calculator uses the standard optical relationships listed above and assumes ideal optics. It does not model glass absorption (use T-stops for that), vignetting or the quirks of a specific adapter. Crop factors are for common sensor formats; enter your own value if your camera crops in a video mode.
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