Almost every vintage SLR lens was made for 35mm film. Put one on a camera with a smaller sensor and you lose part of the picture the lens was designed to make. A focal reducer is an adapter with glass in it that squeezes the lens's full image onto the smaller sensor. It can give you back a wider view and a brighter aperture. It also adds glass and some limits. Here's how they work and when we'd use one.
The crop problem
A full-frame sensor is the same size as a 35mm film frame, so an old lens behaves on it the way it did on film. A smaller sensor only sees the middle of the lens's image circle. The crop factor tells you how much. APS-C sensors are about 1.5 times smaller across than full frame, or 1.6 on Canon's, and Micro Four Thirds is 2.
Multiply the focal length by the crop factor to get the field of view you'd see on full frame. A 50mm lens on a 1.5 crop body frames like a 75mm lens. On Micro Four Thirds it frames like a 100mm. Your normal lens has become a short telephoto, and a 28mm wide angle has become something close to a normal lens.
You also lose the edges of the image circle. For some vintage lenses that's where the character lives. The swirl of a Helios 44-2, for example, grows stronger away from the center, so a cropped sensor shows less of it.
What a focal reducer does
Wikipedia defines a focal reducer, or telecompressor, as an optical element that reduces focal length and increases lens speed. It works the opposite way to a teleconverter. Speed Booster is a trade name for one maker's line, and you'll see sellers use it for the whole category.
Roger Cicala of Lensrentals described it well in a 2013 test. A teleconverter spreads the light leaving the lens so only the center reaches the sensor. A focal reducer compresses that light onto a smaller image circle, so the focal length acts shorter and more light reaches each part of the sensor. The model he tested multiplied focal length by 0.71 and widened the maximum aperture by one stop, turning a 50mm f/1.2 into the equivalent of a 35.5mm f/0.9.
Why 0.71 is about one stop
An f-number is the focal length divided by the width of the lens opening. A focal reducer shrinks the focal length while the opening stays the same, so the f-number shrinks by the same factor. One full stop is a factor of about 1.41 in f-number, which is why the scale goes f/2, f/1.4, f/1. And 1 divided by 1.41 is about 0.71. So a 0.71x reducer makes an f/2 lens act like an f/1.4. Reducers with other ratios exist, such as 0.64x, and they change things by a bit more.
Working out what you'll get
Multiply the focal length by the reducer ratio, then by the crop factor.
- 58mm f/2 on APS-C with a 0.71x reducer. 58 times 0.71 is about 41mm at about f/1.4. Times 1.5, it frames like a 62mm lens on full frame. With a plain adapter it would frame like 87mm.
- 50mm f/1.4 on Micro Four Thirds with a 0.71x reducer. About 35mm at about f/1. Times 2, it frames like a 71mm lens on full frame. With a plain adapter it would frame like 100mm.
On APS-C a 0.71x reducer gets you very close to the lens's original field of view. On Micro Four Thirds it gets you partway, because the crop is bigger than the reducer can undo.
What you give up
You're adding glass between the lens and the sensor. Cicala's tests found the one he tried held up well, with resolution as good or better and brighter corners. He also saw some astigmatism in the corners with one lens, and highlights that bloomed a little at the widest apertures, which went away by about f/2 effective. Adapter tilt showed up as uneven sharpness across the frame.
Wikipedia notes that a lens plus focal reducer has less space behind it, which limits the lenses and cameras a reducer can work with. A lens with a rear element or aperture pin that sticks out far may not fit, so check the reducer's compatibility notes for your lens.
A reducer is also built for one lens mount and one camera mount. If you shoot M42, Canon FD and Olympus OM lenses, you'd need three of them. Plain adapters are small and simple by comparison.
If you buy a used reducer, check it the way you'd check a lens. It has its own glass elements, so ask for a clear photo through it against a light and look for haze, scratches and dust inside.
When we'd use one
- You want the original field of view. If a 50mm lens on your Fuji X body feels too tight and you want it back to a normal lens, a reducer does that.
- You want the edges. For lenses whose look lives toward the frame edges, a reducer brings those edges back onto the sensor.
- You need the extra stop. On Micro Four Thirds, where the crop is biggest, the wider view and brighter aperture together make the most difference.
- You mostly shoot one mount. If most of your lenses are Canon FD, one reducer such as an FD to Micro Four Thirds model covers them all.
When a plain adapter is better
- Full-frame bodies. A full-frame Sony E, Nikon Z or Canon RF body already sees the whole image circle. Use a plain adapter.
- You want the tighter crop. A 50mm on APS-C becomes a handy portrait lens. A 135mm becomes a long telephoto. If that's the point, keep it.
- Small, light lenses. On a light 50mm f/1.8 like the OM Zuiko 50mm f/1.8, a plain OM to Fuji X adapter keeps the setup small.
If you're unsure, start with a plain adapter like the M42 to Fuji X or FD to Micro Four Thirds. Shoot for a few weeks. If you keep wishing the lens were wider or faster, that's when a reducer makes sense. Our methodology page explains how we pick lenses, and you can browse what's for sale now.
