You’re holding an f/1.8 lens and a 0.71× speed booster, and you want one number: what f-number do you actually get? Most people end up doing the multiplication on their phone, in a hurry, between takes. And because 0.71 isn’t a round number, small mistakes creep in. A printed chart fixes that: look up the marked aperture, read across, and move on.
This speed booster ratio chart lists the new f-number and the stops gained for the three ratios you’ll meet most often, 0.71×, 0.64× and 0.58×. You get every full stop from f/1 to f/22, plus the maximum apertures of common lenses. A speed booster ratio is simply the number the adapter multiplies your focal length and f-number by. A 0.71× booster takes f/2 to about f/1.42 and adds roughly one stop. Every figure comes from the same formulas the calculator uses.
You’ll also find the brightness multiplier for each ratio, a quick rule for working out any ratio yourself, and a CSV you can download and keep on your phone. If you want the full explanation of the formulas first, read the guide to speed booster effective aperture, then come back to the chart whenever you need a quick answer on set.
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Table of Contents
- What Is a Speed Booster Ratio?
- Speed Booster Ratio Chart: Stops Gained and Brightness
- What Is the New F-Number at Every Aperture?
- What Do Common Lens Apertures Become?
- Quick Answers About Speed Booster Ratios
- How Do You Work Out Any Ratio Yourself?
- Which Ratio Fits Which Sensor?
- Frequently Asked Questions
What Is a Speed Booster Ratio?
A speed booster ratio is the factor an adapter multiplies your focal length and f-number by. A ratio below 1, such as 0.71×, shortens the focal length and lowers the f-number, which widens the view and brightens the image. A ratio above 1, such as 1.4× or 2.0×, is a teleconverter and does the opposite.
Simply put, a speed booster is a focal reducer, and the ratio tells you how much it reduces. Metabones explains the principle in its Speed Booster white paper: with a magnification of 0.7×, the new focal length is 0.7 times the original, and because the f-number is the focal length divided by the opening, the lens gets faster.
A ratio below 1 is a booster, a ratio above 1 is a teleconverter, and both multiply the f-number the same way.
Speed Booster Ratio Chart: Stops Gained and Brightness
A 0.71× booster adds about +0.99 stops and nearly doubles image brightness (1.98×). A 0.64× booster adds +1.29 stops (2.44×) and a 0.58× booster adds +1.57 stops (2.97×). Here’s the chart, including the two common teleconverter ratios for comparison.
| Ratio | Type | Stops gained | Brightness vs no adapter | Image area vs original |
|---|---|---|---|---|
| 0.71× | Speed booster | +0.99 | 1.98× | 50% |
| 0.64× | Speed booster | +1.29 | 2.44× | 41% |
| 0.58× | Speed booster | +1.57 | 2.97× | 34% |
| 1.4× | Teleconverter | −0.97 | 0.51× | 196% |
| 2.0× | Teleconverter | −2.00 | 0.25× | 400% |
The image area column explains the brightness. A 0.71× booster shrinks the image to 71% of its width and height, so the same light lands on about half the area, and brightness per area roughly doubles. All figures assume ideal optics. Glass in a real adapter absorbs a little light, so the measured T-stop gain is slightly lower.
Brightness rises by 1 ÷ ratio², and the stops follow from that.
What Is the New F-Number at Every Aperture?
Multiply the marked f-number by the ratio. Here’s every full stop from f/1 to f/22 for the three common booster ratios.
| Marked aperture | With 0.71× | With 0.64× | With 0.58× |
|---|---|---|---|
| f/1 | f/0.71 | f/0.64 | f/0.58 |
| f/1.4 | f/0.99 | f/0.9 | f/0.81 |
| f/2 | f/1.42 | f/1.28 | f/1.16 |
| f/2.8 | f/1.99 | f/1.79 | f/1.62 |
| f/4 | f/2.84 | f/2.56 | f/2.32 |
| f/5.6 | f/3.98 | f/3.58 | f/3.25 |
| f/8 | f/5.68 | f/5.12 | f/4.64 |
| f/11 | f/7.81 | f/7.04 | f/6.38 |
| f/16 | f/11.36 | f/10.24 | f/9.28 |
| f/22 | f/15.62 | f/14.08 | f/12.76 |
Notice that the smallest apertures shift too. Stop a lens down to f/22 behind a 0.71× booster and you’re really at about f/15.62. If you want the chart in a spreadsheet, download the speed booster ratio chart as a CSV. It covers 31 apertures from f/0.7 to f/22, including the in-between third-stop values.
Every row is the marked aperture times the ratio, top to bottom.
What Do Common Lens Apertures Become?
An f/1.8 lens becomes f/1.28 with a 0.71× booster, f/1.15 with a 0.64× booster and f/1.04 with a 0.58× booster. These are the maximum apertures you’ll find on most lenses people adapt.
| Lens maximum aperture | With 0.71× | With 0.64× | With 0.58× |
|---|---|---|---|
| f/1.2 | f/0.85 | f/0.77 | f/0.7 |
| f/1.4 | f/0.99 | f/0.9 | f/0.81 |
| f/1.8 | f/1.28 | f/1.15 | f/1.04 |
| f/2 | f/1.42 | f/1.28 | f/1.16 |
| f/2.8 | f/1.99 | f/1.79 | f/1.62 |
| f/3.5 | f/2.49 | f/2.24 | f/2.03 |
| f/4 | f/2.84 | f/2.56 | f/2.32 |
| f/5.6 | f/3.98 | f/3.58 | f/3.25 |
Values below f/1.0 are real optics, but your camera may not show them. Metabones explains in its FAQ that its adapter reports the boosted aperture only for lenses of f/1.8 or slower. With an f/1.4 lens or faster, the screen keeps showing the original value even though the exposure has changed. Other adapter brands may behave differently, so judge exposure from the image or a meter.
For fast lenses, the new f-number can be lower than what your camera displays.
Quick Answers About Speed Booster Ratios
What Is the Difference Between a 0.71× and a 0.64× Speed Booster?
Simply put, a 0.64× booster shrinks the image more than a 0.71× booster, so it adds more light and a wider view. It gains about 1.29 stops against 0.99 for the 0.71×. The trade-off is coverage: the smaller image circle has to fit your sensor, which is why the stronger ratio is made for smaller sensors. It matters most when you choose between the two for a Micro Four Thirds camera.
Speed Booster Ratios at a Glance
| Aspect | Details |
|---|---|
| Symptom | You need the new f-number without running the maths every time you swap lenses |
| Root Cause | The ratio multiplies the f-number, and 0.71, 0.64 and 0.58 are not round numbers |
| Fix | Read the chart row for your marked aperture, or multiply the f-number by the ratio |
| Performance Gain | +0.99 stops at 0.71×, +1.29 at 0.64×, +1.57 at 0.58× (slightly less in T-stops) |
| Applies To | Any lens and focal reducer pair, with ideal optics |
When Does This Apply?
It applies whenever you place a focal reducer (ratio below 1) or teleconverter (ratio above 1) between a lens and a camera. The numbers are theoretical f-stops for ideal optics. They don’t include light the adapter glass absorbs, so confirm exposure with a meter.
How Do You Work Out Any Ratio Yourself?
Three formulas cover every ratio: new f-number = marked f-number × ratio, stops gained = −2 × log2(ratio), and brightness multiplier = 1 ÷ ratio².
Here’s a custom ratio of 0.726 on an f/2 lens, step by step:
- New f-number: 2 × 0.726 = f/1.45.
- Stops gained: −2 × log2(0.726) = +0.92 stops.
- Brightness: 1 ÷ 0.726² = 1.90×.
There’s also a shortcut. A ratio of 0.707 (1 ÷ √2) is exactly one stop, so any ratio near 0.7 is about one stop. A ratio of 0.5 is exactly two stops. That’s why 0.71× adds 0.99 stops, just under a full stop, and why you can sanity-check any figure in your head.
One stop sits at 0.707 and two stops at 0.5, so every other ratio falls in between.
Which Ratio Fits Which Sensor?
0.71× is the general-purpose ratio, and stronger ratios such as 0.64× are for smaller sensors. A stronger ratio gives more light and a wider view, but it shrinks the image circle further, and the sensor has to stay inside it.
Suggestion of Motion’s comparison of the Metabones Ultra (0.71×) and XL (0.64×) shows how this plays out on a Micro Four Thirds camera. For the sensor sizes, the combined crop factors and a coverage check, see the speed booster crop factor table.
Pick the ratio for your sensor first, then read the new f-number from the chart.
Frequently Asked Questions
How many stops does a 0.71x speed booster add?
About 0.99 stops, which is almost exactly one stop. The exact one-stop ratio is 0.707 (1 ÷ √2), and 0.71 sits just above it. A 0.64× booster adds about 1.29 stops and a 0.58× booster adds about 1.57.
What f-number does f/1.8 become with a 0.71x speed booster?
About f/1.28, from 1.8 × 0.71 = 1.278. With a 0.64× booster it becomes f/1.15, and with a 0.58× booster f/1.04. The lens opening doesn't change. Only the focal length and the f-number do.
What does a speed booster ratio of 0.58 mean?
The adapter multiplies focal length and f-number by 0.58. That adds about 1.57 stops and shrinks the image circle to 58% of its width. It suits small sensors, because the shrunken circle still has to cover the sensor.
Is the speed booster ratio the same as crop factor?
No. The ratio belongs to the adapter and the crop factor belongs to the camera. Multiply them to get the combined crop factor, for example 2.0 × 0.71 = 1.42 on Micro Four Thirds.
Can I use this ratio chart for a teleconverter?
Yes. A ratio above 1 multiplies the f-number up instead of down. A 1.4× teleconverter turns f/2.8 into f/3.92 and loses about 0.97 stops, and a 2.0× teleconverter loses exactly 2 stops.
Why does my adapter's real gain look smaller than the chart?
The chart assumes ideal optics. The glass in the adapter absorbs a little light, so the measured T-stop gain is slightly below the f-stop figure. Check the maker's T-stop data when it's published, and confirm exposure with a meter.
Do the f-number changes depend on the sensor size?
No. The new f-number depends only on the lens and the ratio. The sensor changes the framing, because the camera's crop factor multiplies on top, so the chart works for any sensor as long as the image circle covers it.
Can I download the speed booster ratio chart?
Yes. The CSV on this page lists 31 apertures from f/0.7 to f/22 for 0.71×, 0.64× and 0.58× boosters. It opens in any spreadsheet app, so you can keep it on your phone for quick checks on set.
That’s the speed booster ratio chart in one rule: multiply the marked f-number by the ratio. A 0.71× booster adds about one stop, a 0.64× booster about 1⅓ stops and a 0.58× booster about 1½, and the tables above give the exact f-number for every full stop and for the lenses people adapt most. Check the numbers against a meter, because real adapters lose a little light.
It’s one more piece of the lens-maths picture: how ratio, crop factor and aperture combine when you adapt lenses across sensor sizes. Adapters and prices keep changing, so check the maker’s current spec sheet in 2026 before you buy, and trust your own meter over the number on the screen.
For a ratio that isn’t on the chart, open the speed booster effective aperture calculator and use the custom ratio field. Bookmark this page for the chart, download the CSV for your phone, and share it with the friend who keeps asking what their adapter does to the aperture.