Focal Reducer
An adapter containing glass that compresses a lens's image circle onto a smaller sensor, shortening focal length and gaining light.
Formula / rule
f_new = f × reduction factor
Measured in
× reduction
In depth
Often called a speed booster, it is the optical inverse of a teleconverter: a 0.71× reducer turns a 50 mm f/1.4 into a 35 mm f/1.0 equivalent. The gain is real, not marketing — light that would have fallen outside a crop sensor is redirected onto it. It only works when adapting a lens designed for a larger format than the camera's sensor.
Examples in practice
A full-frame 85 mm f/1.8 on an APS-C body with a 0.71× reducer behaves like a 60 mm f/1.3.