Zhongyi Speed Boosters are a cheaper version of what Metabones have been providing for years. i.e. They 'widen' the view of the lens and allow around 1 stop more light onto the sensor, meaning narrower apertures and / or higher shutter speeds. Now the price difference between Metabones and Zhongyi is quite significant. So is the Zhongyi any good? Well I can't comment on the long term robustness of it, but after having it a few weeks I can say, it's well made, works well and does what it's supposed to. The one I have is for Nikon lenses. I have a selection of MF lenses with aperture rings, but G lenses can be used because there is an aperture adjustment ring on the adapter, though the travel from wide open to maximum aperture is a very short travel and there are no guides to let you know what aperture you are selecting.
I took it out yesterday specifically to take some photographs of the interior of a stately home, which are usually dark and poorly lit. I used my Voigtlander Nokton 58mm f/1.4 II, which when attached to my Panasonic GH5 via the Zhongyi adapter approximated to an 85mm f/1 on 35mm / 'FF'. (With all the usual provisos about 'equivalents'). However it was a pretty fast lens.
All of the images above are straight out of the camera jpgs. with no editing whatsoever. All were shot at f/1.4 (f/1 ??) using the Zhongyi adapter and the GH5's focus peaking. I would say that the pictures give a false impression of the amount of light I had to work with. Iy was pretty dark in there. Only one picture was shot at ISO 3200, the rest at lower settings, usually in the 800-2000 range. All are excellent in terms of dynamic range, noise levels and sharpness. Below is the one image which required ISO 3200.
As you can see, it's an excellent result and certainly my stock pictures would accept this without a problem.
So with a combination of Panasonic's improved high ISO output and using a speed booster with a fast lens, virtually all of the issues around m4/3 and low light have been eliminated. Sure. anything above ISO 6400 is a bit nasty, but how many of us m4/3 users need those very high ISO values anyway. If we do, then I guess we wouldn't be using m4/3 and choose something more appropriate. But for general use and including pro. uses such as wedding photography, the high ISO capabilities of the GH5 are perfectly adequate for even the most demanding of users. The elimination of noise and retention of sharpness are very welcome.
The more I use it, the more I'm impressed with the GH5. Panasonic have certainly come up with with the best m4/3 camera ever, in my opinion and one that can be classed as a genuine allrounder. I am a little surprised at how good it actually is. I bought it for a combination of 4K video and good stills shot mostly in good light. However, it is a good deal more versatile than that. And it's a mirrorless camera with those advantages.
Incidentally, Kirk Tuck has some good articles on the GH5 and in particular one on the Nikon D850. Now I was going to write a post on the D850, but I thought since Kirk pretty much summed up what my views are I would just give the link. The D850 is I'm sure an impressive camera (though the rave reviews about it are somewhat premature, since nobody has used it seriously for any length of time, apart from the usual bought and paid for Nikon hacks) but like many, I'm just so used to mirrorless now and it's ease of use for video, EVF's and all the other stuff that comes from a genuine hybrid camera. I don't dislike DSLR's, but they aren't what I want to use right now. And for the video capability they offer, they are a compromise. That SLR mirror is becoming more and more redundant as time goes by and while pro news and sports shooters may still rely on those big and heavy Canon and Nikon workhorses, for many of the rest of us there are much better options, for what we do, with mirrorless cameras.