I saw posted some patent applications from Olympus and Panasonic on lens mount adaptors for 4/3rds. What's interesting about them is that they include a focal length reducer of .5x. This is cool because it restores the correct field of view to legacy 35mm lenses while simultaneously speeding them up to provide equivalent apertures. It's reported that Olympus essentially used such a formula grafted onto the back of their f/2 zooms. That is they used an f/2.8 full frame design with a .5x reducer on the back, and then closed the aperture down a bit to f/2 because they couldn't quite make it f/1.4 without some problems. People who've played around with them say there's a bit more than f/2 aperture available by fooling with the lens release mechanism.
Panasonic's version is interesting because it provides for a .5x focal reduction and a mirror-prism for phase detection, so, in condensing the light by a factor of .5x, you create a 2 stop brighter aperture. Same field of view and same depth of field. A 24-70 f/2.8 on 35mm, through the reducer becomes a 12-35 f/1.4 on m4/3.
Please note, this is by no means a more compact set-up. It's even bigger (lens plus reducer) but it gives you the same exposure through a smaller sensor, at least in theory.
Why would anyone want this? Cinema applications with expensive cinema lenses comes to mind. As a gateway camera from DSLR to EVF cameras, but perhaps something a bit expensive for that? For a future with a sort of sensor size independence?
Same could be done, for example, using APSC sensors and a .67x reducer. A 24-70 f/2.8 full frame becomes a 16-46.6 f/2 APSC, again with the same depth of field and field of view.
I don't see it happening, but it is interesting. A looooong time ago, Nikon tried this built into a camera, but it was a huge frankencamera, and probably the focal reduction was too great. Curiously, the optical reduction system sucked up a lot of light (the effective T-stop of the lens-camera combination being a lot less) which is not my understanding of how a reducer should work - ie. concentrating the same light into a smaller area; thus larger relative aperture...
Anyway, interesting nonetheless. Maybe I'll update this post with some links later.
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