I recently bought the Metabones EF-RF Speed Booster. A lot of people have commented on the Canon and Viltrox EF-RF speed boosters, but only a few people have talked about Metabones' offering on the RF mount, even though it's been on the market for a few years now. This is not a scientific review. I have used the Speed Booster, and I can say the image quality is perfectly good (though obviously with more glass you're always introducing the potential for new aberrations). But I don't have any of the competing products myself, so I can't do any direct comparisons (I've tried to read through the reviews/look at samples for the competitors, but I have not tested them myself.) I also don't have any charts or anything to show you. If there is interest, I can try to go out and generate some samples if you want them (I don't normally share my work on threads here because of the privacy aspects, but if I shot images just for DPR forums, that'd be fine.)
Why use a speed booster? I got the booster mostly because I was interested in using my ultra-wide FF lenses on the R7. I sometimes use my R5 and R7 at the same time; I was recently using my 14-24mm f/2.8 on my R7 and thought I could use a wider field of view for my video from that camera. I could have gone for an EF-S 10-22mm or 10-18mm, but those lenses are f/3.5 and f/4.5 at 10mm, respectively. For around the same price as an EF-S 10-22mm f/3.5-4.5, why not go for a speed booster then (for around the same price) and make my FF 14-24mm f/2.8 an APS-C 10-17mm f/2.0?
My main use — using full-frame lenses on a smaller sensor — is the classical use for a speed booster. But when the Metabones Speed Booster was brought to the RF mount, there was not yet an APS-C RF camera. There are some uses for the booster on full-frame RF cameras. The EOS R shoots 4K only in a ~1.7x crop mode, which was the first major application of this Metabones Speed Booster. If you use an EOS R, that may make this accessory useful. On the R5, shooting 5K RAW in cropped mode may be useful (either because of a smaller file size, 60fps RAW or because it is compatible with the original Atomos Ninja V in this mode). And so on.
The exact crop factor you get will depend on your camera and mode. Metabones offers this handy guide to the crop factors you'll get with various different setups.
(A little note, because this can confuse people sometimes. The speed booster does not actually change any properties of your lens. My 14-24mm f/2.8 will show as a 10-17mm f/2.0, but the actual properties of the lens have not changed. The difference is that the image circle has been projected onto a smaller space than usual, just like an extender projects the image circle onto a larger space. This is really useful for packing an image large enough for a full-frame sensor so that most of it can be seen by a smaller APS-C camera sensor. The displayed values are the equivalent ones: the lens now functions as an APS-C 10-17mm f/2.0 lens. But this doesn't change how much light is physically let in through the lens' front element. You will not get more light on your APS-C sensor/in crop mode than you would get on a FF sensor in full-frame mode. Exceptionally, you will get more light on your sensor by using more of the image circle as described in a later section, but this is usually pretty severely limited by the size of the image circle your lens projects.)
Why the Metabones? I'd heard good things about Metabones' 0.71x boosters on other mounts, and the EF-RF 0.71x booster reportedly shares its optics with those. Canon's (slightly more expensive) speed booster is only fully compatible in firmware with the C70, and so use on other RF cameras requires passing through a warning screen and potential future oddities.
Viltrox pulled their speed booster off the market when they pulled their RF lens from sale. I would prefer to buy a product currently on the market and getting consistent firmware support. In addition, I'd looked into reviews of the Viltrox booster and got the impression that it was perhaps of worse optical quality than Metabones on various other mounts. (I have absolutely no concerns about Metabones pulling their adapter; tons of companies offer EF-RF adapters, including filter adapters, speed boosters, etc., with no issues; I suspect Viltrox pulling their booster and adapter had to do with some settlement relating to the AF lens and not related to these products themselves, but anyway, if you want to discuss that, there are threads for it.)
Besides Canon and Metabones, there is also a Venus Optics Laowa EF-RF speed booster, but this is only designed for their probe lenses, and doesn't include any electrical contacts (since their probe lenses are fully manual). I wasn't interested in buying that.
Some Canon and Viltrox speed booster reviews/comments both noted that these focal reducers could result in a purple circle in flare in backlit scenarios. Obviously this doesn't occur in all circumstances, and of course it has to do with the optical design of these boosters. They don't have the same design, but they do both reportedly exhibit this problem sometimes. Since I didn't see anyone (I could have missed something) complain about the Metabones 0.71x Ultra Speed Booster exhibiting this issue on other mounts (and the optics for their 0.71x are the same across mounts), I figured that this point might be better with Metabones than Canon (or Viltrox).
Finally, the Metabones Speed Booster ($480) is cheaper than Canon's offering ($600), though not as cheap as Viltrox's (currently selling online for like, $300?) and Laowa's ($250) were/are (plus, it's still current and has electronic contacts).
Build quality and physical attributes. The speed booster feels solid. It has a metal construction and feels nice in the hand. The mounts aren't weather sealed, however. It fits quite snugly on the camera and lens mounts, with as little play as I've felt on any adapter. (The lens mount on my copy actually feels if anything a slight bit rigid — not in a way that I ever felt posed a problem for my lenses, but just not being quite as smooth as a Canon adapter is, with there being a little bit of a metal-on-metal feel as you turn the lens into locked position.)
The speed booster comes in a nice little hard-plastic carrying case which looks and feels fine but which I'll probably forego for my camera bag, personally.
The booster comes with a tripod foot attached. It can be removed from the adapter with a little hex key (not included).
Firmware and updating. Like I mentioned, Metabones, unlike Canon, officially supports all RF cameras with their firmware (Canon cameras and the RED Komodo), not just cinema cameras.
In terms of firmware updates, I am pretty reassured by Metabones' track record. They seem to update their products fairly well. This particular RF speed booster has been updated a number of times, in order to correct some firmware compatibility issues that arose on the R5/R6 and C70 when they first came out, as well as to make other tweaks to enhance support for various lenses. (It's worth mentioning that most cameras haven't required adjustments from Metabones in firmware upon release — the R3/R7/R10 didn't for instance — so it's probable that it'll work with the rumored RF cameras at launch.)
The firmware correctly integrates with Canon's menu system. If you go to the screen on your camera to check your firmware version, it'll actually display it (along with the camera and, if applicable, lens firmware version). This showed that my speed booster shipped with firmware 1.8.0 (which is still current). Updates, however, are not installed through the Canon updater, but instead through the Metabones utility on your computer. The firmware can be not only updated, but also downgraded back to an older version if you'd like (the option is officially supported).
The connection is made through a standard Micro-USB cable. I would have preferred USB-C at this point, but Micro-USB is still valid. The speed booster doesn't come with a cable, but you most likely have one already — you just might have to go look for it for a minute if you're now almost entirely on USB-C like I am. The Micro-USB port on the speed booster is covered by a little cap which is easy to take off and put back on but which could probably be easily lost if you left it off.
Interestingly — and this I only learned by reading the firmware update notes — the Metabones adapter includes special firmware support for the 1980s weird Sigma lenses that had a poorly implemented version of the EF protocol and thus don't allow for electronic aperture control on newer Canon bodies ("newer" meaning since like sometime during the 1990s). I happen to own a Sigma 400mm f/5.6 lens from the late 1980s which suffers from this defect (it's an ancient lens, I got it for almost free, and no, it's not optically good but that's besides the point — it's a toy). It works properly on the booster! Amazing!
Use for the intended purpose. The speed booster works successfully on my R5 and R7. It translates the metadata of the lens to the speed-boosted equivalent focal length and aperture, except that it will not lower the f-stop of any lens whose maximum aperture is already f/1.4 or wider. The lenses have full electronic control and autofocus seems to be exactly as it is (no better, no worse) when the lenses are used without the speed booster.
The booster is supported for both stills and video. By default (but see below), the booster will make the lens identify itself as APS-C only, and so the R5 applies the 1.6x crop mode, just like using an EF-S lens on a Canon adapter would.
The booster has a little wheel on it that can be used as an RF control ring (programmed however you want, as usual). The wheel can be pressed in, which is also programmable in-camera (as a lens button). Using the Metabones utility on your computer, you can also set pressing down on the control wheel to set EXIF data you pre-specify, which may be useful if you are using an adapted vintage lens (like with Nikon F-EF on top of EF-RF), or a cinema lens/other full-manual lens with no electronics.
In my testing, the speed booster has worked quite well for both photos and videos, with everything being really quite as you'd expect in normal operation in terms of autofocus/electronic control/etc. The image quality also looks pretty good. Like I said, I have not conducted and form of scientific test. But the images coming from this unit are pretty good. Since the optics are supposed to be the same, perhaps any review of the Metabones 0.71x Ultra Speed Booster on other mounts could provide more potentially useful information.
Also, while I obviously cannot say definitively that there would never be any flare issues, I can say that my tests with backlighting did not produce a purple circle in any backlit situations like described for the Canon and Viltrox speed boosters. Not saying that there couldn't be a flare issue in some situation, but I wasn't able to see any problem with flare myself.
Bonus fun stuff! Ah, but wait, there's more! In the Metabones utility on the desktop, you can check a box to set whether or not the speed booster will force an APS-C crop. By default, it does, but if you uncheck this option, then you can use your camera in full-frame mode with a speed-boosted lens! (The APS-C mode still works exactly as before, but your camera won't automatically switch into it like with the default settings.)
What use is there for this wizardry? Well, it will give you the 0.71x reduced image from your speed booster but on the full sensor of your full-frame camera. You should expect any vignette (as this is effectively attaching a full-frame lens to a camera with what is now an 0.71x crop factor). All the lenses I tested had a significant vignette on the R5 in this mode. But some of them had a bit less than most: the Sigma Art prime lenses were in general better than the other lenses I tested, especially the 50mm and 40mm lenses. You can get significantly more than you bargained for with these lenses in terms of image circle! But do keep in mind that this purpose is really entirely secondary to the main use of the adapter, and it's mostly something you can play with for fun rather than something which produces good images for real use (as you're using the glass for a purpose for which it was not designed, after all).
You may be able to get a simulation of an 0.71x medium format camera using this adapter with another adapter that attaches a medium-format lens to an EF mount. I don't have any medium-format lenses and I don't have such an adapter so I have no actual experience testing that kind of setup. There is also a Kipon speed booster for that purpose (https://www.youtube.com/watch?v=u5VwtZThygs), but I've never used that either.
Conclusion. If you want a speed booster for RF cameras, I think this is a pretty good option. It's certainly worth considering, at least, if you're in the market for that. If you are shooting video (or stills!) on an R7/R10/other future APS-C RF camera, and want closer to full frame FOV and DOF (although not quite the same as what you'd get on FF, to be clear), this speed booster may be for you. If you want to use one of the very nice 4K-crop (or 5K-RAW crop) modes on the full-frame RF cameras, this may be for you, too. If you want to get right about full frame on the C70 or slightly more on a Komodo, this may be what you're looking for. (On the other hand, if you're shooting stills on a FF RF camera, you probably shouldn't buy a speed booster, unless you... really really want to capture your RAWs in 1.6x crop mode for some reason and throw away tons of pixels, or if you really want to get a bit more of the image circle than you're supposed to.)
