Sony Alpha 7R II can match or beat DSLR low light AF performance
High-end DSLRs have dedicated phase-detect AF (PDAF) modules that collect a lot of light per AF point, often functioning reliably in conditions as dim as -2EV or even -3EV. Most mirrorless cameras in low light, though, would often exhibit the hunting characteristic of contrast-detect AF (CDAF), either because of lack of on-sensor PDAF or because in dim conditions, the camera would revert to CDAF.
As such, a major question among people debating mirrorless vs DSLR is: can mirrorless cameras focus in as dim light as DSLRs?
When the Sony Alpha 7R II was announced, it promised a lot of game-changing features, but its low light AF capability was an unknown. We've already seen some of the incredible continuous AF abilities of the a7R II, including pinpoint eye AF for even moving subjects, but a lot of pros may be wondering: 'if I ditch my DSLR for some of the advanced AF features the a7R II offers, will I be sacrificing low-light AF performance?'
Well, we decided to investigate this, and the results may surprise you:
Bright lenses allow the a7R II to continuously focus in -2EV
And there you have it: when paired with bright lenses, the a7R II can match, and in our tests here even exceed, the low light performance of DSLRs known be the best in terms of low-light focus ability. Even in continuous drive, the a7R II kept up with the D750 and 5DS at -2EV (0.6 lux as measured by our scientific meter), if not slightly outperformed them by shooting at a higher effective frame rate. But how?
Part of the reason the Sony can perform so well is that we're using a lens with an F1.4 maximum aperture. The DSLRs' focus modules essentially create 'virtual apertures' that only 'see' small regions from around the lens (read this excellent in-depth treatise by Douglas Kerr). They essentially can't see light rays entering peripheries of the lens beyond a certain aperture, which means that they can't take full advantage of the lenses' brightness in these dark conditions. Meanwhile, as far as we know, the masked pixels that comprise Sony's on-sensor AF elements see half the lens at all times, so can see the full F1.4 aperture. Therefore, the camera can take advantage of all that extra light these fast primes bring, not just in terms of exposure, but in terms of focus too.
If there are reports of poor low light AF ability with the a7R II floating around the internet, it's likely because of the slow lenses attached by those testing and making these claims of poor performance. Of course, slow lenses may be what you shoot, in which case you likely won't see the extremely impressive low-light focus performance we're showing here, but if you're a pro or someone who shoots a lot in dim conditions, we think it's safe to say you'll be shooting primes at least some of the time... in which case you should see that the a7R II actually catches up to the best DSLRs in terms of its low light AF ability. And it maintains this performance across most of the frame.1 That's no mean feat.
Sony a7R II | FE 35mm F1.4.
AF-C vs AF-S
Something to note: we've been consistently finding that AF-C (continuous) mode tends to shorten the hunt and feedback process that we see in AF-S mode, which gets even longer in low light (the AF system needs to work slower to collect more light). Therefore, you'll likely get snappier results in low light in AF-C, with one exception which we'll get in to below. Note though that if it's so dark you're experiencing hunting (below -3EV), you'll want to switch to AF-S so that once your camera locks focus, it doesn't try to refocus which may send it in to a long hunt. Why the hunt? Because in very dim conditions with low contrast subjects, the PDAF sensors may cease to function, at which point the camera reverts to CDAF. This is, of course, better than giving up entirely, as CDAF will continue to function when PDAF fails.
In fact, it's CDAF that gives the a7S its low light AF sensitivity, which we measured to focus in conditions even dimmer than Sony's claimed -4EV, below the point at which the a7R II will give up.2 However, we found this incredible sensitivity of the a7S to practically be of limited use for quick shooting or moving subjects, because of the slow speed due to the lack of PDAF. When it comes to the a7R II, it's really PDAF that makes it shine: in -2EV conditions and brighter, the a7R II paired with a bright F1.4 or F2 lens is snappier at focusing than the a7S. In other words, AF-C is really where the a7R II shines.
There's an exception though. Because of the way the Sony controls its aperture, it doesn't always take full advantage of its own ability. When you have the camera set up to give you a preview of your live exposure (which is how most people will probably want to work with mirrorless cameras), the camera stops down the lens to the user-chosen aperture. Only at the time of focus acquisition does the camera open up its aperture to let in more light and reduce depth-of-field for faster focus acquisition.3 Here's the thing, though: we often noted the camera would sometimes only open up to F2, and only when it got really, really dark did it open up our F1.4 lens all the way. Furthermore, in AF-C mode, if you've selected a shooting aperture of, say, F2.8, the camera initially acquires AF wide open, but then immediately stops down the aperture to your selected aperture in preparation for your shot, which then reduces the amount of light available to the AF elements.
Sony tells us this is done to ensure the camera is ready to shoot, the moment you hit the shutter button. However, we're concerned that this push to minimize shutter lag risks decreasing the chance of the camera achieving focus, which seems like a more pressing issue, at least to us. Therefore, to guarantee best results in dim conditions, you'll want to be shooting at or near wide open. This isn't a huge deal, since in dim conditions you probably want to let in as much light as possible anyway, but we'd like to see Sony address this in a firmware update. Especially because this behavior means that if you've selected F9 or smaller as your shooting aperture, and you're in AF-C mode, your camera just gives up PDAF altogether, since phase-detection stops working at F9.
The a7R II, when paired with a bright lens, can match or exceed the performance of the best DSLRs with respect to low-light AF ability. And we haven't even shared the most exciting thing with you yet: although in this video we show you results at -2EV (which looks this dark on an iPhone 6 Plus), our 'extinction' tests measured -3EV as the failing point of phase-detection with the a7R II + 35mm F1.4 combo. That's very dark, and represents potentially game-changing performance, in our opinion.4
Sony a7R II | FE 35mm F1.4
We have to say that when we started this test, we weren't expecting these results. But in retrospect, after sharing our exciting findings with Sony engineers, we're not too surprised. An AF engineer we spoke to claimed that they'd measured the ability of the a7R II to focus down at -2 EV with a F2.0 lens. Coupling this with our insight regarding on-sensor AF vs. DSLR AF, and the ability of the former to take advantage of the extra light of bright lenses, things fall in to place. -2EV capability with a F2.0 lens means -3 EV capability with a F1.4 lens - makes sense, doesn't it? For the most part, you'll see an increase in low light AF performance if you pair your mirrorless camera with a brighter lens. We were even able to get the a7 II to focus down to -3EV.
What's particularly exciting is when you factor in the established benefits of on-sensor focusing when it comes to focus accuracy, compared to DSLRs. When it comes to fast primes and tight focus tolerances, missing AF can be more the norm than the exception with DSLRs. What's more, the optimum calibration value (Canon AF 'microadjustment', Nikon AF 'Fine-Tune') can actually vary based on things like color temperature of the light, ambient temperature, subject distance, and more.
On-sensor PDAF implementations obviate all these issues. Imagine shooting that 85mm F1.8 prime and actually having it focus on what you want it to focus on, no front-focus, no back-focus. Or just hit 'Eye AF' and have it focus automatically. On moving subjects. Even in low light. In fact, in all my shooting of late with the a7R II and F2 primes, I've yet to find a low-light situation where the camera resorted to hunting. I wish I could say the same about my time with my Nikon D810 (you did watch to the end of the video, didn't you?).
 We verified this by testing all cameras using off-center points as well. Only the 5DS' center AF points demonstrated this level of low light sensitivity, and while all of the D750's points focused down to -3 EV, consider that its frame coverage is markedly smaller than that of the a7R II's PDAF system.
 Keep in mind that the a7S also benefits from brighter lenses, which allowed it to focus below -4EV in our tests. This enhanced sensitivity of the a7S CDAF system also means that even when it's paired with slow F4 lenses, it can compete with the low light AF ability of the a7R II with faster lenses (albeit at the cost of slower AF due to CDAF).
 Kudos to Sony for at least doing (fixing) this, as previous Alpha series cameras would often not bother to open up their apertures for focus acquisition until it was absolutely pitch dark, which meant that for most dim conditions, the cameras would slow down their own AF systems when you'd selected a smaller shooting aperture. This was a complaint we lodged against those cameras time and again, and we're glad to see it fixed.
 Remember that -3EV focus capability doesn't mean the benefits are only relevant at -3EV conditions; instead, better low light sensitivity means better, faster performance in even less dim conditions, particularly with lower contrast subjects where a less sensitive AF system might struggle to find enough contrast to make focus measurements.
Acknowledgements: Thanks to Richard Butler for his help in making the video, and Sam Spencer for being a kind, patient soul.