I've just bought a Metabones 0.71x Speed Booster ULTRA to use my Canon EF lenses on my Panasonic GX8's. This gives the one stop light boost as usual and makes a 0.71 crop for the lenses as opposed to the usual 2x. This means that my Canon 'Full-Frame' lenses will be slightly wider than they would be in front of an APS-C sensor. In addition to this, the Metabones has an electronic connection with the lens which means it a) autofocuses b) allows me to change the aperture electronically c) triggers the lens IS if it has it and d) provides a proper exif readout.
After using it for a day a few things nave become obvious. This is without doubt the most amazing piece of photographic gear I've ever bought. The lenses all focus really quickly, about the same as they focus on my Canon 5Ds itself. The Metabones extra lens, focusing a large lens area onto a smaller sensor means that the image quality is superb. As good as the best m4/3 can offer and in many cases better. m4/3 owners (assuming your camera can accept this, not all can) now have almost the entire range of Canon lenses at their disposal, with almost full functionality, including AF and aperture selection and all with 1 stop better light gathering than they achieve on Canon cameras. It is simply a stunning adapter. It's not cheap (over £500) but for me well worth it because - I am considering after only a day with it selling pretty much everything (except see** below)) except my Canon 5Ds body, my Canon EF lenses and my two Panasonic GX8 bodies, one of which I'm setting up for stills and the other for 4K video (for which I will be getting more Metabones Speed Boosters, including the 0.64x one,) because this makes real sense, both in terms of a massive reduction in the gear I have to own and of course the money I have tied up in cameras and lenses that don't get used much.
To be honest. I'm amazed at just how good this adapter is. See below for some 100% blowups plus the exif data the adapter generates.
The Canon lens will also work in the new Panasonic post focus mode via the Metabones. It gives a range of focus points and I can select the frame(s) I want.
Above shows what the combined lens and body IS is capable of for hand held video. This is using the 24-105mm f/4 zoom. Note the last clip with the two cars at the longest end of the zoom. I've not got better than this from 5-axis IBIS.
Just so you get an idea as to what I'm ending up with, below is a list of the Canon EF lenses I have currently. In the table below you will see the lens as it will be used on my Canon DSLR, what it approximates to in terms of using it on a m4/3 cameras via the Speed Booster and the 'full-frame' approximation of how that m4/3 adaptation mimics the behaviour of the lens on a larger sensor.
Approximate 'equivalents' for my Canon EF lenses.
Canon 17-40mm f/4
Canon 24-105mm f/4 IS
Canon 24mm f/2.8 IS
Canon 40mm f/2.8
Canon 85mm f/1.8
m4/3 'equivalent' specs.
17-75mm f/2.8 IS
17mm f/2 IS
'FF equivalent' on m4/3
35-150mm f/2.8 IS
35mm f/2 IS
As you can see this provides me with some amazing options and of course cheaper ones than if I bought 'full-frame' lenses with those specs.. With the speed boosting all those lenses get 'faster' on m4/3. Now as I made clear before, the apertures don't actually change, but there is a one stop boost for either the shutter speed or the ISO setting. This instantly gives my GX8's a one stop boost in terms of how they perform in low light as well.
**Now this does mean I'm a bit short on wide-angle options for m4/3. So unless I can find a wider option (and I'm going to investigate some of the wide Sigma W/A zooms like the 8-16mm and 10-20mm) I'll hang on to my Samyang 14mm (Nikon fit) plus Nikon > m4/3 0.68x Speed Booster until (and if) I can find a better alternative. And since my Nikon lenses work on the Canon DSLR, that's not really a problem.
FIRST IMPRESSIONS CONCLUSION
I've never been someone who likes brand exclusivity as I feel that it's restrictive. Currently the two best non-MF lens ranges we can buy are Zeiss Otus and the Sigma ART series. Both are independent lens makers. Now Canon make decent, if not the very best, lenses, but the optics of the speed booster offer a slight improvement in performance as well as the 1 stop boost and 0.71x crop. This makes the adapter very useful indeed and as indicated above, I'm seeing results from my EF lenses that compare very well with the best that m4/3 can offer, which is pretty remarkable. So in effect Metabones and Brian Caldwell are offering something special and while, as ever, I would recommend getting a 'second opinion' and hopefully trying this out for yourself, I think a case can be made for m4/3 camera owners contemplating the purchase of Canon EF lenses plus this adapter instead of using native m4/3 lenses. Certainly using the S/H market it is possible to get faster, cheaper lenses that the somewhat overpriced top end Panasonic and Olympus offerings. Though of course, there are size and weight issues to consider.
I'll leave it there for today, because to be honest I'm keen to get back to trying this little wonder out, but as you might expect there will a lot more on this. And I have to say it's once again respect to Brian Caldwell who came up with this concept. I had an email conversation with him a couple of years ago and I know he is a m4/3 user, so this is not something that has happened in isolation, but is designed by a photographer for photographers. Plus it wasn't on the list of nominations for Soundimageplus accessory of the year and it is very late in the year, but it's come flying up on the rails down the home straight to win by some distance. Brilliant.