DriftProductions

You have to go into stills mode, do single-shot AF-S on your subject then hit the video record button (with 1080/120p set in the menus)
Whilst it is recording, AF won't track the subject.
I agree with the OP, Sony should try and get phase-detect AF working with 120fps... Can't see why it shouldn't!

Hello guys !
After a few days using the rx100 v in the lovely city of Prague for a long weekend, I decided to keep this little camera. The overheating issue was non-existent there (of course it was freezing outside and I used the manual mode/single autofocus which seems to work better for better battery life and less overheating) and the footage you can shoot with is amazing. I'll try to post some soon. Once again the continuous autofocus is so snappy and convenient for that type of situations (grabing short shots on the fly).
My post is more of a request to Sony. I noticed that in 100/120fps mode, you don't have access to the autofocus settings (sensitivity, speed) and apparently the phase detection is disabled. It makes the autofocus in that mode sooooo slow ! Even slower than what you had with the mk IV in 25fps. No face detection, no nothing. It's very unfortunate I think. I hope Sony could fix it with a firmware update because the autofocus is so slow that it almost looks like a bug. The only thing that makes me think it's not, is the autofocus settings greyed out in that mode. I wanted to do slow mo/gimbal shots with my rx100 and I'll be limited to 60fps for now (the autofocus works in 50/60fps).
So please Sony, make this wonderful autofocus work with 120fps mode !
 

Thanks @Andrew Reid your advice is appreciated  Most of what you and others had mentioned i'd already learnt from regularly reading your posts and forum members feedback from the forums over this last year or so, i've learnt so much in a short space of time and it's been invaluable really. Gonna stick with my plan, save up the pennies and run with the Sony A7s in crop mode and eventually get a speedbooster when the budget allows. 
Haha you are like the 'Annoying Orange' mate!  I'l be sure to be extra specific next time, keep the peace yeww
Thanks

@j_one couldn't have said it better, bang on! I'm still gonna stick with the A7s and wait for a price drop. Cheers mate. 

FWIW, regarding the BMMCC/Metabones, I just set up a new rig with those. haven't shot enough with it but enough to really like it with the smallrig/ikan EVF setup we're using. Very affordable, really exceptional IQ, slow motion color science and dynamic range and the peaking is some of the best I've used. I shoot full frame Canon's as well, but I can tell this will be a go-to combo.  Here's a quick first test of the slomo...nothing special.
https://vimeo.com/202145227
 

If you say so. Imo it would have been a non issue if one read the post before answering it. But maybe that's just me.

D750 in 1080/60p is sharper than the A7S in full frame 60p mode.
But APS-C mode A7S gives better 1080/60p than almost anything else for the money and you have way more choice of lenses on E-mount than Nikon F-mount.
Sadly no Speed Booster
Not sure what happened to the custom one on this forum.
So this is $999 used... anything cheaper?
Cheapest would be a used Panasonic G6... 1.3x crop with Speed Booster and Canon lenses, so nearly full frame.
Check out the quality of 1080/60p on the Fuji XT-20 though... and you can put a Nikon mount Speed Booster on it, but not Canon.
How about forgetting about 60p altogether and going for a nice 120p RX100 IV if it is slow-mo you're looking for?

Oh yeah, if you're cool with hacks, the best option IQ wise, imo
5d mkiii RAW 10bit 60p 
Wedding example

Imo the three point bold lettered bulletin list was enough.

Keep your eye on Ebay.
Sometimes, especially at odd times like say a Friday after a Thursday holiday or other non normal times you might find a used A7s selling to Australia only with little competition.
I have had mine for a couple of years now but that is how I got mine.     It was actually sealed in box but was technically sold second hand and for a fair bit less than I otherwise would have had to pay.
I also have a GX7 that I really like but it has a few too many limitations for video for me.      Maximum ISO of 3200 for video and no mic or headphone jack being the biggest.
That said, I don't do a lot of video beyond recording the occasional song at rock/blues gigs and I am more often than not over ISO 3200 (sometimes way over) so the GX7 will never work for me for that nine times out of ten.
A7s is not the camera for everyone but I think it is worth every cent they go for.
EDIT    A quick check of Ebay Australia  sold listings has several first version A7s cameras having gone in the range of $1572 including postage to around 2100 or so but many of the higher priced ones have extras with them.     Plenty in the range 16-1800 range.    Some have lots of bids, some only two or three.

Here in NZ you can buy used A7S for NZD$1400 2nd hand, there was a few a7s sold for $1500, dunno why but A7II still hold value steady while a7s took a tumble
Here in NZ you can buy used A7S for NZD$1400 2nd hand, there was a few a7s sold for $1500, dunno why but A7II still hold value steady while a7s took a tumble

Okay, this one doesn't fit your speedbooster to full frame criteria, but if I was on a budget and looking for good 60p I would just go with the Nikon D5500 or 5600. The 7200 offers zebras and metering with ai/ai-s lenses but the 60p has an added crop, so the D5500 or 5600 is probably a better fit. The Flat profile is amazing and APS-C 60p may hold you over until the A7Siii arrives. Another option would be a D500... 4K with a crop but 1080p 50/60p with APS-C and EIS. It seems to be a beast of a camera. 

NX1 still best value for money, now with a speedbooster too!
They don't make them as they used too..
 

Let me respectfully suggest you're in a "forest for the trees" situation.
If you want super cheap, and are truly strapped for cash, then maybe step back and consider older LUMIX cameras, speedbooster, and a old fast manual prime lens for low-light situations.   I've shot numerous documentaries and projects with a variety of LUMIX M4/3 cams.  As far as I'm concerned, the 60p on these cameras is fine.  I really have an affinity for my GX7, for example.  An effective S35 sensor area is adequate.
I mean, we're talking about $500 to get all three pieces of gear; even less if you buy used.  No, they're not the best low-light cameras @60p, but do-able and that price is ridiculous considering what you can accomplish.
If you don't like the results it's not like you've invested a lot of cash.  With those cameras, buy used and then sell used and you won't lose any money regardless.
Ultimately it'll come down to your abilities with whatever gear is in hand.  Certainly don't assume you are going to solve the majority of various production problems just because you own a camera that's 2 stops faster than another one.  I mean, low-light-full-frame cameras are fun and all, (I do like them too) but work is work, your budget is tight, and cameras are only a small fraction of the whole process.
All that said: NX1 and X-T20 tick boxes what you want, just at a higher price.

True, my AF100 is quite old, but as great in camera 60 fps slow motion (recorded to 24 fps, so you don't have to fiddle with speed on your editor of choice).
The gh4 is 1080p 60 fps (and more) and is quite affordable now

The problem seems to be that most can't do full sensor readout at more than 25 fps, so you can either get great 4K footage from downsampling the sensor (A6300/6500), or lineskipped 1080p at up to 4x speed. Seems to be a next generation thing, as the NX1 (ahead of its time in many ways) and GH5 seem to have faster readout, so it is probably the next thing to come to Sony, though APS-C upgrades will probably have to wait till after the next FF releases. Perhaps the A7iii?

GH5? 4k 60p, downrez ro 1080p should be amazing
 

I hate to be "the NX1 guy" in this thread ha (I'm actually suprised none of the usual suspects have piped up yet)... But it might be worth looking into the NX1, or the even cheaper NX500 for that matter, both produce lovely, sharp 60p (and gorgeous 120p as well!) with minimal moire/aliasing, crazy low rolling shutter, and decent enough lowlight performance with a fast lens, which you're obviously interested in, i.e. shallow dof, or you wouldn't care so much about sensor size. I have no idea what they'd go for down under, but there are lots of gently used ones around for pennies. Our very own forum member Lucabutera is selling some awesome NX-L speedboosters that he has crafted as well, I have one, which gives the camera about a 1.1x crop. 
Seems like it'd fit your parameters pretty spectacularly. Best of luck! 

I have been hoping the original A7s drops also, but I gave up holding my breath! It still is, in certain circumstances, ergo low light,  the way to go for the money. Not the best overall for the money I guess, but I would still kill for one LoL. And 1080p looks great on it also.

Hey Geoff, cheers for the suggestion, already considered, but a used D750 nearly costs as much as a new A7s here in Australia. Hoping to find something cheaper, closer to the $1000 price point if a camera even exists at this price... 
Agreed about the 1080p output. There's such good value even in the cheapest cam's like the Panasonic G7/G85 packed full of killer features with a beautiful 4K image, and once downscaled to 1080 look amazing... the menus, image stabilisation, focus peaking, ease of use, everything is unreal in these cams. Yet camera companies still can't produce an entry level camera with decent 1080 50/60p slow-mo at a reasonable price point despite most people delivering their content in 1080.
I do wonder how long will it take for full frame cameras to become affordable? I can't do camcorders mate, so might have to hang out for the next release of the A7s iii series and wait for the A7s to drop in price. Thanks

For some reason most of the 1080p output on DSLR's, Mirrorless FF cameras, even APSC that I have seen that have 4k on them seem to suck, no clue why, but 4k looks great down sampled to 1080p. So slow motion on them is not much to write home to mother about is what I am saying. Not many cameras in that sector have 4k slo mo over 30p.
Seems dedicated Camcorders are the only thing that has great 1080p with slo mo I like.
So used cameras like a Sony F3 or FS100,  older Panasonic camcorders, are my picks.

The Metabones Speedbooster XL is dope! I shoot with the G7, the XL Canon mount and Sigma 18-35mm f1.8 and the results are killer for what I need. Example...
 

Not my thing. I so rarely use things like that so owning it doesn't make much sense. That's why Im looking forward to the upcoming smaller version...

BTW, Zhiyun has a smaller version coming up.

Just bought one used for AUD$450, without having the camera balanced properly its already flying better than my Came Mini. Really impressed, ergonomics are a bit weird though. I think this one is finally a keeper. Started with the nebula 4000, its amazing how far things have come in a year