Articles · 16 September 2026 · 6 min read

Speed boosters vs plain adapters: what the glass does

A speed booster is an adapter with glass inside: a wider view and about a stop more light, but only onto a smaller sensor. How it differs from a plain ring.

A plain adapter is an empty tube that holds a lens at the right distance from the sensor; it changes nothing about the image. A speed booster (the generic name is focal reducer) is an adapter with a lens group inside that shrinks the lens’s image circle onto a smaller sensor. That makes the field of view wider and the effective aperture brighter, typically by about one stop for a 0.71× reducer. It only works when adapting a lens made for a larger format onto a smaller sensor.

What a plain adapter does

A plain adapter contains no glass. Its only job is to fill the gap between the lens’s flange focal distance and the body’s, so the lens sits exactly where it was designed to. Put a Nikon F lens (46.5 mm flange distance) on a Nikon Z body (16.0 mm) and the adapter is a precisely machined 30.5 mm spacer, with or without electronic contacts.

Because nothing optical is added, the lens behaves as it would on its own mount: same focal length, same aperture, same sharpness. On a smaller sensor you get the usual crop, so a full-frame 50 mm lens on a Micro Four Thirds body frames like a 100 mm lens on full frame. The adapter pages on this site, such as Nikon F lenses on Nikon Z, are all plain adapters of this kind.

What a focal reducer does

A focal reducer places a positive lens group behind the adapted lens. It gathers the image the lens projects and compresses it into a smaller circle. Lensrentals’ early analysis put it simply: the Speed Booster “compresses the light leaving the lens onto a smaller image circle”, which makes the focal length appear shorter and concentrates the light onto less area.

Two things follow.

The focal length shortens by the reduction factor. A 50 mm lens behind a 0.71× reducer behaves like a 35.5 mm lens. On an APS-C body that largely cancels the crop: Metabones says its 0.71× Speed Booster ULTRA takes the crop factor of APS-C cameras such as Sony E and Fujifilm X “from 1.5x to 1.07x”.

The aperture gets brighter by the same factor. The same light lands on a smaller area, so the effective f-number is the lens’s f-number multiplied by the reduction factor. Light per unit area scales with the square: 1 ÷ 0.71² is roughly 2, which is one stop. At 0.64×, 1 ÷ 0.64² is about 2.4, or about 1⅓ stops. Metabones rates its 0.64× XL at 1⅓ stops, with an f/1.2 lens becoming f/0.80.

Real reduction factors

These are products whose factors are stated by the manufacturer:

The XL’s 0.64× on Micro Four Thirds brings the 2× crop factor down to 1.28×, in Metabones’ words turning the camera into “APS-H format” in field of view.

Why it only works onto a smaller sensor

A focal reducer shrinks the image circle. That is only useful if the lens’s original image circle was larger than the sensor to begin with.

• Full-frame lens onto APS-C or Micro Four Thirds: the lens covers far more than the small sensor needs, so the reducer can compress that surplus onto it. This is the intended use.
• APS-C lens onto APS-C: there is no surplus. The shrunken circle would no longer cover the sensor and the corners would go dark. Metabones states plainly that a full-frame lens is required and that EF-S, DX, DC and Di II lenses are not supported by the ULTRA 0.71×.
• Full-frame lens onto full frame: same problem, which is why there is no focal reducer for full-frame bodies.

There is a second constraint: space. The reducer’s glass has to sit between the lens and the sensor, so the camera needs a short flange distance and nothing in the way. Metabones’ FAQ gives the reason a Speed Booster cannot work on a DSLR: “The mirror gets in the way”. Short-flange mirrorless bodies are what made the product possible, which is why they arrived for Sony E, Fujifilm X and Micro Four Thirds and not for SLRs.

Image quality trade-offs

Adding glass changes the image, not always for the worse.

• Centre sharpness can hold up or improve. Lensrentals’ bench tests of the original Speed Booster found centre resolution slightly better than through a plain adapter, with comparable corners.
• Wide open, it shows. The same tests found highlights tended to “bloom a bit at the widest apertures” and saw astigmatism in the corners, which largely cleared by f/2.
• Quality depends on the adapter. Metabones advertises multi-element designs using high-index glass; cheaper reducers use simpler optics. Results vary with both the reducer and the lens behind it.
• More surfaces, more chances for flare. Any extra glass is another place for stray light to bounce.

A plain adapter has none of these effects. If the lens was good on its native mount, it is equally good on a plain ring.

Autofocus through a reducer

Electronic reducers pass autofocus through much as electronic plain adapters do, with the same caveats covered in autofocus with adapted lenses. For Canon EF on Micro Four Thirds through the Metabones 0.71×, this site rates autofocus as single-shot only and slow. Metabones also notes that AF on the XL fails if the lens (plus any extender) is slower than f/8 before the boost.

Which one to buy

• You want a full-frame lens to behave as it did on full frame, on a crop body: a focal reducer is the only way to get the wider view and the extra stop.
• You want the extra reach of the crop, for wildlife or sport: a plain adapter. The crop is the point.
• You are adapting onto a full-frame body: a plain adapter. A reducer will not cover the sensor.
• The lens is an APS-C design: a plain adapter. Reducers are not built for these lenses.

For the Metabones EF-to-Micro Four Thirds reducer: Search Amazon UK.

Where to go next

• Flange focal distance explained covers why short-flange bodies have room for a reducer.
• The adapter index lists every pairing this site covers, plain and optical.
• The mount hubs give flange distances and formats for each system, including Canon EF, the most commonly reduced lens mount.