TIL: In 1994 Kodak patented a focal reducer for film SLRs

Whenever the topic of focal reducers (AKA speed-boosters) comes up, the question of why no one makes a Nikon F to DX or Canon EF to EF-S adapters is often raised. The prevalence of adapters for MFT and other mirrorless bodies, and the apparent similarity to teleconverters is often brought up.

In the past, I've often said that you can't design a focal reducer for like mounts, because reducing the focal length requires moving the lens closer to the image sensor. It turns out that this answer is only partially true.

In fact, Kodak patented a general purpose focal reducer in 1994. But because the focal reducer had to act as a retro-focal lens, the absolute aperture of the system was limited and the adapter had to have incredibly complex optics.

For anyone interested in the topic, Meta Bones has a very interesting white-paper. In the white paper, they explain how a speed-booster works, and why it works best when converting 35mm SLR lenses for use on mirrorless crop bodies. It discusses how the search for a good general-purpose focal reducer inspired their creation of the Speed Booster. It also explores the challenges facing previous attempts based around like-for-like mounts.