Speed boosters & focal reducers,
explained
▸ See a boosted build
A teleconverter makes a lens longer and darker. A speed booster does the opposite: it's an adapter with a glass element that shrinks the lens's full image circle down onto a smaller sensor. The result — a wider field of view and about a stop more light, at the same time. It's the closest thing to cancelling crop factor.
The booster factor is less than 1 — usually 0.71× or 0.64×. So the lens frames wider, and because the same light is packed onto a smaller area, it also gets faster:
| Booster | A 50mm becomes | Light gained | Effect on a 2.0× M4/3 body |
|---|---|---|---|
| 0.71× → | ~35.5mm | +1 stop | 2.0× → effective ~1.43× |
| 0.64× → | ~32mm | +1⅓ stops | 2.0× → effective ~1.29× (≈ Super 35) |
focalcomp resolves the whole chain — lens, booster, sensor — and shows the effective focal in the readout, so the math is never guesswork.
You need a larger-format lens. A booster mounts full-frame (or EF) glass onto a smaller sensor. You can't boost a lens that already matches the sensor.
Coverage matters. The lens's image circle has to be big enough to reduce. Full-frame glass on a Micro Four Thirds or Super 35 booster is the sweet spot.
Optics quality varies. Metabones and Viltrox are excellent; the cheapest generic reducers can soften the corners.
The right booster depends on your body and lens mount. Our hands-on guide ranks the three worth buying for the most popular booster body, the Blackmagic Pocket 4K.
▸ Best speed boosters for the BMPCC 4KDoes a booster hurt image quality?
Good ones don't — concentrating the image circle can even sharpen the center. Only the cheapest reducers soften corners. Stick to reputable brands and you gain a stop for free.
Speed booster vs plain adapter?
A plain adapter only solves the mount (it keeps the crop). A booster has glass that reduces the image — wider and brighter. That's the difference between a $30 ring and a $200–650 optic.
Can I boost to full frame?
Not quite — most boosters land you near Super 35 (a 0.64× on Micro Four Thirds). True full-frame recovery would need a stronger reduction than the optics allow while staying sharp.