LensMath calculators use standard optical relationships and nothing hidden. This page lists every formula, the assumptions behind it, what the results leave out and how errors get corrected, so you can check any number yourself. The calculator is version 2.2, built and maintained by Mubeen.
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The Formulas
The ratio is the number printed on the adapter. Below 1 it’s a focal reducer (speed booster), such as 0.71× or 0.64×. Above 1 it’s a teleconverter, such as 1.4× or 2.0×. Every result comes from these relationships:
- 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)
- Estimated T-stop (optional) = effective aperture × 2loss ÷ 2, where loss is the adapter light loss in stops that you enter
- Net light change (optional) = light change in stops − adapter loss
- Ratio you need (ratio finder) = target full-frame focal length ÷ (focal length × camera crop factor)
- Full-frame equivalent ISO (optional) = ISO × camera crop factor². It uses the camera’s crop factor only, not the booster ratio, because the sensor size sets the noise comparison
Exposure follows the effective aperture. Depth of field follows the full-frame equivalent aperture. The guide to speed booster effective aperture walks through each step with examples.
How Do the Results Compare With Published Figures?
They match. Take a 50mm f/1.8 lens with a 0.71× booster on a 1.5× APS-C camera. The calculator returns 35.5mm and f/1.28 for the lens, 53.25mm and f/1.92 in full-frame terms, and +0.99 stops of light.
Manufacturer figures agree with the same arithmetic. DPReview’s report on the Metabones Speed Booster ULTRA lists a 1.42× crop factor for a full Micro Four Thirds sensor (2.0 × 0.71) and a 50mm lens becoming 35.5mm. The Metabones FAQ multiplies 0.71 by a 1.52× crop and gets about 1.08×, which is the 1.0792 the calculator gives for the same inputs.
Inputs, Limits and Rounding
| Input | Allowed range | Note |
|---|---|---|
| Focal length | 1 to 3000mm | The original lens focal length |
| Aperture | f/0.1 to f/64 | The original lens f-number |
| Ratio | 0.3 to 3 | Presets, or a custom value such as 0.726 |
| Crop factor | 0.1 to 10 | Picked from the camera list or typed in |
| Adapter light loss | 0 to 3 stops | Optional. Leave blank for ideal optics |
| ISO | 1 to 1,000,000 | Optional. Shows the full-frame equivalent ISO |
| Target full-frame focal length | 1 to 6000mm | Ratio finder only |
The calculation uses full precision. Results are rounded to two decimal places for display, with exact halves rounded up (2.485 shows as 2.49), and trailing zeros are removed (35.50 shows as 35.5). The ratio finder shows ratios to three decimals. If the crop factor is 1 or lower and the ratio is below 1, the calculator shows a vignetting warning, because a speed booster shrinks the image circle and a full-frame sensor may not stay covered.
Where Do the Camera Crop Factors Come From?
The camera list uses typical crop factors for each sensor format: 1.5 for Sony, Nikon and Fujifilm APS-C, 1.6 for Canon APS-C and 2.0 for Micro Four Thirds. Crop factor is the full-frame diagonal (about 43.3mm) divided by the sensor diagonal. Exact sensor sizes vary slightly between models, and video modes can crop further, so type your own value if you know it. The speed booster crop factor table shows the combined figures for common ratios.
What Do the Results Leave Out?
The maths assumes ideal optics, so the results are theoretical values. They don’t include:
- light absorbed by the glass in a specific adapter, unless you enter the loss yourself (the calculator can’t know your adapter’s real figure)
- vignetting or edge softness from a particular lens and adapter pairing
- video crop modes that change your camera’s effective crop factor
Always check exposure with a meter or the histogram in real shooting conditions. Our disclaimer covers the limits in full.
Is Any of My Data Sent Anywhere?
No. The calculation runs in your browser, and the numbers you type are not sent to a server. The Copy link button only builds a link on your device. It contains your values, so share it only if you’re happy for others to see them. The privacy policy explains the analytics the site does use.
Corrections and Updates
If a result looks wrong, send the lens, ratio, camera and the number you expected through the contact page. Confirmed errors are fixed and recorded in the changelog below.
Changelog
- October 2026: calculator version 2.2 added an optional ISO field with a full-frame equivalent ISO, and the calculator can now start from different defaults on other pages. Added a guide to APS-C to full frame conversion.
- October 2026: calculator version 2.1 added the ratio finder, the optional adapter light loss field with an estimated T-stop, and a Copy link button. Also added a speed booster ratio chart with a CSV download, guides on whether speed boosters add light and on crop factor with a speed booster, a guides index, and this methodology page.
- September 22, 2026: launched the speed booster effective aperture calculator with effective aperture, effective focal length, full-frame equivalents, light change in stops and teleconverter ratios.
Frequently Asked Questions
Is the speed booster calculator accurate?
It's exact for the formulas it uses, and it agrees with published figures such as the 1.42× crop factor for a 0.71× booster on Micro Four Thirds. Real lenses and adapters differ slightly because the maths assumes ideal optics, so confirm exposure with a meter.
Does the calculator include T-stops?
Only if you enter them. By default it gives theoretical f-numbers. If your adapter maker publishes a light loss, type it in to see an estimated T-stop and net light change. Real gains are slightly lower than ideal figures because adapter glass absorbs some light.
Where do the camera crop factors come from?
They are typical crop factors for each sensor format: the full-frame diagonal of about 43.3mm divided by the sensor diagonal. Models vary slightly, so enter your own crop factor if you know the exact sensor size or recording mode.
Does the calculator send my lens data anywhere?
No. The calculation runs in your browser and nothing you type is sent to a server. The privacy policy explains the analytics the site does use.