LensMath guides explain what a speed booster (focal reducer) does to your lens: how much light you gain, what your crop factor becomes, and how every calculator result is worked out. Every guide uses the same formulas as the calculator, so the numbers always match.
New to focal reducers? Read the effective aperture guide first, then run the calculator on your own lens.
Start With the Calculator
Speed Booster Effective Aperture Calculator: enter your lens, booster ratio and camera crop factor to get the effective aperture, effective focal length, full-frame equivalents and light change in stops. It runs in your browser.
Light and Exposure
- Speed Booster Effective Aperture: How to Calculate It: the formula, worked examples, full-frame equivalents and answers to common focal reducer questions.
- Do Speed Boosters Really Add Light?: separates exposure, total light and depth of field, so you can see why people argue about it.
- Speed Booster Ratio Chart: the new f-number and stops gained for 0.71×, 0.64× and 0.58× at every aperture, with a CSV download.
Framing and Crop Factor
- Speed Booster Crop Factor: APS-C and MFT Table: the combined crop factor for common sensors and ratios, which ratio fits which sensor, and equivalent focal lengths.
- APS-C to Full Frame Calculator: convert focal length, aperture and ISO to full-frame equivalents, with and without a speed booster.
How the Numbers Are Calculated
- LensMath Methodology: every formula, the assumptions behind it, what the results leave out and how errors get corrected.
Which Guide Should You Read?
- Is the extra stop real? Read do speed boosters really add light.
- What f-number will I get at each aperture? Open the speed booster ratio chart.
- What is my lens on full frame? Use the APS-C to full frame calculator.
- What will my lens frame like? Read the speed booster crop factor table.
- Why does the calculator give this number? Read the methodology page.
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