You mount a full-frame lens on a Micro Four Thirds or APS-C camera with a 0.71× speed booster, and the f/1.8 on the barrel suddenly behaves like f/1.28. Your shutter speed doubles. Then you open a forum thread that calls the extra stop a myth, and another that calls it plain physics. Both sound convincing, and that’s the problem.
So, do speed boosters really add light? Here’s the thing: both camps are usually right, just about different things. “More light” can mean three separate ideas. One is how bright the image is on the sensor. Another is how much total light the lens collects. The third is how shallow the depth of field looks. A speed booster changes the first, leaves the second alone, and changes the third only once you name what you’re comparing it with. Even Metabones, the company behind the Speed Booster name, says depth of field has a short answer and a long, complicated one.
This page pulls the three ideas apart with numbers you can check yourself. You’ll also get the quirks that trip people up in practice: the minimum aperture shifting too, camera screens showing the wrong f-number, and why a T-stop isn’t an f-stop. If you want the formulas first, start with the guide to speed booster effective aperture, then come back here for the argument.
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Table of Contents
- Do Speed Boosters Really Add Light?
- Why Does Everyone Disagree? Compared to What?
- How Does a Speed Booster Lower the F-Number?
- What Do You Gain in Real Exposure?
- What Does a Speed Booster Not Change?
- Does a Speed Booster Make Depth of Field Shallower?
- Quick Answers About Speed Boosters and Light
- F-Stops, T-Stops and Real-World Loss
- Why Does My Camera Show the Wrong Aperture?
- Frequently Asked Questions
Do Speed Boosters Really Add Light?
Yes for exposure, no for total light. A 0.71× speed booster makes the image on the sensor about one stop brighter, so you can use twice the shutter speed or half the ISO. It doesn’t make the lens gather more light overall, because the front opening stays exactly as wide as before.
Simply put, a speed booster raises the density of light on the sensor, not the amount of light the lens collects. Brightness goes up by the ratio, and total light stays where it was.
Why Does Everyone Disagree? Compared to What?
Most speed booster arguments end the moment someone names the baseline. The same adapter gives a different answer depending on what you compare it with, so let’s break this down in one table.
| Question | With a 0.71× booster on APS-C | Compared with |
|---|---|---|
| Is the image brighter? | Yes, about +0.99 stops | The same lens on the same camera, no booster |
| Does the lens collect more total light? | No, the opening is the same size | The same lens before adding the booster |
| Is depth of field shallower? | Yes at the same framing, no at the same distance (about 1.4× either way) | The same lens without the booster |
| Is the f-stop gain real? | Yes, the f-number at the sensor drops by the ratio. The T-stop gain is slightly smaller | The aperture marked on the lens |
Before you decide who’s right in a thread, ask what each person is comparing against.
How Does a Speed Booster Lower the F-Number?
Simply put, the f-number is the focal length divided by the diameter of the entrance pupil, the effective opening of the lens. A speed booster cuts the effective focal length by its ratio and leaves the opening alone, so the f-number drops by the same ratio.
Take a 50mm f/1.8 lens. Its entrance pupil is about 27.8mm (50 ÷ 1.8). Add a 0.71× booster and the effective focal length becomes 35.5mm. Divide by the same 27.8mm opening and you get f/1.28. The glass opening didn’t change. Only the focal length did.
The ratio applies to the whole aperture scale. f/2.8 becomes about f/1.99, f/4 becomes about f/2.84, and f/22 becomes about f/15.6. For other ratios, the speed booster ratio table lists the new f-number at common apertures.
Does the Minimum Aperture Change Too?
Yes. Metabones notes that its Speed Booster boosts the minimum aperture as well as the maximum. Stop a lens down to f/22 behind a 0.71× booster and you’re really at about f/15.6. If you need very small apertures, for example for long exposures in daylight, expect to reach for an ND filter or remove the booster. So the whole aperture scale shifts by the ratio, top and bottom.
What Do You Gain in Real Exposure?
The gain in stops is −2 × log2(ratio). That’s +0.99 stops at 0.71×, +1.29 stops at 0.64× and +1.57 stops at 0.58×.
Geometry explains the number. A 0.71× booster shrinks the image to 71% of its width and height, so the same light lands on about half the area (0.71 × 0.71 ≈ 0.5). Brightness per unit area roughly doubles, and exposure follows brightness per unit area.
You can spend that extra stop in two ways:
- Keep the ISO and use a shutter speed one stop faster, for example 1/100 s becomes 1/200 s.
- Keep the shutter speed and drop the ISO one stop, for example ISO 800 becomes ISO 400.
It’s not just theory. In its first impressions test of the Metabones Speed Booster, DPReview found that a Canon EF 40mm f/2.8 behaved like an f/2.0 lens for exposure, which let the photographer shoot at a lower ISO in dim light. That matches the +0.99 stop calculation.
For exposure, a 0.71× booster is worth about one stop, whichever way you spend it.
For every aperture at once, see the speed booster ratio chart
What Does a Speed Booster Not Change?
A speed booster doesn’t make the front opening of the lens bigger, so the total light entering the lens at a given shutter speed stays the same. Simply put, exposure measures light per area, while noise depends on the total light.
Think about it this way: ISO and exposure describe how bright the image is per unit of sensor area, not how much light the sensor received in total. A smaller sensor behind the same lens collects less total light, and a booster hands part of that back by using light a plain adapter would waste.
Compare the setup with the same lens on a full-frame body at the same framing, and the total light comes out about the same. A 0.71× booster on a 1.5× APS-C camera has a combined crop of about 1.07, so it gets close to a full-frame lens, not past it. This is the idea behind photographic equivalence, which Joseph James explains in detail: the same field of view, aperture diameter and shutter speed give the same total light, and with equally efficient sensors, similar noise.
The booster closes most of the gap with full frame, but it doesn’t push past it.
Does a Speed Booster Make Depth of Field Shallower?
It depends on what you hold constant. At the same framing, depth of field gets shallower. At the same subject distance, it gets deeper. Both shifts are roughly 1 ÷ 0.71, or about 1.4×, using the standard depth-of-field approximations.
Say you shoot a 50mm f/1.8 lens on an APS-C camera. Without a booster the view is 75mm and the depth of field matches f/2.7 on full frame. With a 0.71× booster the view widens to 53.25mm and it matches f/1.92. Step closer until the subject fills the frame the same way as the unboosted shot, and the boosted version has about 1.4× shallower depth of field, which is the ratio of f/1.8 to f/1.28.
Now stay where you are. The wider view lowers the magnification, so the boosted shot looks deeper by about the same factor. That’s the situation behind the claim that boosters make depth of field wider. Both statements are true. They just describe different comparisons.
Metabones adds a third comparison. A 50mm f/1.4 behind a 0.71× booster becomes roughly a 35mm f/1.0 lens, and that always has a shallower depth of field than a straight 35mm f/1.4 at any distance. Same focal length, bigger opening.
Depth of field only looks shallower once you reframe to the same field of view.
Quick Answers About Speed Boosters and Light
What Causes the “Is It Real?” Confusion?
Simply put, people use “more light” to mean three different things and judge each one against a different baseline. The three are brightness on the sensor, total light collected, and the depth-of-field look. A booster changes the first by the ratio, leaves the second alone, and shifts the third depending on framing. It matters most when you compare a boosted crop-sensor camera with a full-frame body or a native lens.
Speed Booster Light Gain at a Glance
| Aspect | Details |
|---|---|
| Symptom | Conflicting claims about whether the extra stop is real |
| Root Cause | “More light” means brightness, total light or depth-of-field look, each judged against a different baseline |
| Fix | Name the baseline first. Use the f-number for exposure, the aperture diameter for total light and the full-frame equivalent aperture for depth of field |
| 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 | Mirrorless cameras with a sensor smaller than the lens image circle, such as APS-C, Super 35 and Micro Four Thirds |
When Does This Apply?
It applies when you adapt a lens made for a larger sensor, such as full frame, to an APS-C, Super 35 or Micro Four Thirds mirrorless camera. It doesn’t apply to native crop-sensor lenses. Boosters also aren’t made for DSLRs or full-frame bodies, where the mirror or the sensor size gets in the way.
Pros and Cons of Using a Speed Booster for Extra Light
- Pro: about one stop more exposure at 0.71×, for a faster shutter or lower ISO
- Pro: recovers most of the field of view the crop factor takes away
- Pro: shallower depth of field at the same framing
- Con: extra glass absorbs some light, so the real gain sits a little under the maths
- Con: quality varies by adapter, and cheaper models can soften edges or add flare
- Con: works only on mirrorless cameras and lenses with an image circle bigger than the sensor
F-Stops, T-Stops and Real-World Loss
Simply put, an f-stop is a calculation from focal length and opening size, while a T-stop measures the light that actually gets through after the glass absorbs some. The extra glass in a booster absorbs a little, so the real gain is slightly under the +0.99 stops on paper.
Does Only the T-Stop Improve?
Some people claim that only the T-stop improves and the f-stop stays the same. Metabones argues that’s self-contradictory: a T-stop can’t be faster than the f-stop, so you can’t measure a T-stop gain without a real f-number gain behind it. Either way, the difference you see in your footage is the exposure difference. For cinema work, check the T-stop figure from the adapter maker when one is published.
Treat the f-stop figure as the ideal and the T-stop as what you’ll actually meter.
Why Does My Camera Show the Wrong Aperture?
Because some adapters don’t report the boosted value for fast lenses. Metabones explains that its Speed Booster shows the boosted aperture only when an f/1.8 or slower lens is attached. With an f/1.4 or faster lens, the camera shows the original aperture even though the real one is faster. Its reasoning is that many cameras behave erratically with apertures below f/1.0, and the EXIF standard doesn’t define them. Other adapter brands may behave differently.
A 50mm f/1.4 behind a 0.71× booster is about f/0.99, whatever the screen says. The boost is still there optically and in auto exposure, so judge exposure from the image or the histogram. And be careful when you transfer settings from a handheld light meter, because the displayed number won’t match the real aperture.
The displayed aperture can lag behind the real one, but the exposure doesn’t.
Frequently Asked Questions
Does a speed booster change the lens aperture?
It changes the f-number at the sensor, not the physical opening. The aperture diameter stays the same, but the shorter effective focal length lowers the f-number by the ratio, so a 0.71× booster turns f/2 into about f/1.42.
Does a speed booster improve low-light performance?
It lets you use a faster shutter speed or a lower ISO, about one stop at 0.71×. That helps in dim scenes. The lens still gathers the same total light, so the gain is measured against the same lens on the same camera without the booster.
Do speed boosters reduce noise?
Only indirectly. The extra stop lets you lower the ISO or speed up the shutter, which can help. Against the same lens on a full-frame body at the same framing, total light and noise stay similar, because the opening doesn't get bigger.
Is a speed booster worth buying just for the extra stop?
Usually not on its own. One stop is modest, and a small light or a faster native lens can add more. A booster earns its place when you also want the wider field of view from full-frame glass on a smaller sensor.
Does a speed booster change bokeh?
Metabones reports a negligible effect: the bokeh is mostly that of the lens alone, and the adapter leaves no signature of its own. What changes is depth of field, which gets shallower at the same framing.
Why does my camera still show the original aperture?
Some adapters only report the boosted value for slower lenses. Metabones, for example, shows it with f/1.8 or slower lenses and the original value for faster ones. The boost still happens optically and in auto exposure, so trust the image, not the displayed number.
Does a speed booster change the smallest aperture too?
Yes. The ratio applies across the whole aperture scale, so f/22 becomes about f/15.6 with a 0.71× booster. If you need very small apertures, use an ND filter or remove the booster.
How many stops does a 0.64x speed booster gain?
About 1.29 stops, from −2 × log2(0.64). A 0.58× booster gains about 1.57 stops. The ratio table on the homepage lists the new f-number at common apertures.
So, do speed boosters really add light? They add brightness, about one stop at 0.71×, and that gain is real, measurable and usable. They don’t add total light, which is why an APS-C or Micro Four Thirds camera with a booster gets close to a full-frame lens but doesn’t beat it. Name the baseline before you argue, and most of the disagreement disappears.
This is one piece of the wider equivalence picture: how focal length, aperture and crop factor combine when you adapt lenses across sensor sizes. Adapters and prices keep changing, so check the manufacturer’s current spec sheet in 2026 before you buy, and trust your own meter over the number on the screen.
To see the numbers for your own lens and camera, open the speed booster effective aperture calculator. Bookmark this page for the three-question breakdown the next time the debate starts, and share it with the friend who swears the extra stop is a myth.