The three phones can capture video up to 2160p in resolution, though it's just the 6T that can do it at both 60fps and 30fps - the other two can only do 4K30. Of course, all three can shoot 1080p at both frame rates.
The 3T has a particularly dumb way of changing resolution as it cycles them upon tapping the resolution button and you don't immediately know whether it'll go higher or lower. Good thing OnePlus changed that and the 5T and 6T are more logical in this respect giving you an actual menu.
The video bit rate in 1080p/30fps is 20Mbps on all three phones, while the 2160p footage is treated to 56Mbps on the 3T and 42Mbps on the 5T and 6T. The 6T's 2160p/60fps clocks in at a whopping 120Mbps. All three phones record stereo audio, with the 3T and 5T encoding it at 96kbps, while the 6T allocates a more generous 156kbps.
There's the matter of stabilization. The 3T's lens is stabilized and it can add to that electronic stabilization in 1080p, but not in 4K. The 5T and 6T can electronically stabilize 2160p footage as well, and the 6T's lens is stabilized too, unlike the 5T's.
What we have a problem with is that OnePlus doesn't let you turn off the stabilization in the modes it's available in, regardless of model. That effectively means that you'll be getting a cropped field of view (because that's how electronic stabilization works) and that absolute sharpness will be lower in the modes where there is stabilization, whether you need it or not.
This becomes abundantly clear in our balcony clips taken on a tripod, where the sharpest 4K footage comes out of the 3T and the 6T's 60fps mode. Now, the 3T has other issues, namely the limited dynamic range, but then again the 5T isn't much better in this respect either, plus it's softer than ideal. Which brings us to the 6T and its dramatic improvement in dynamic range, plus it's also not half bad in terms of detail. Still, going 60fps will give you that extra bit more sharpness. As for colors, being fully aware that it wasn't the liveliest of scenes, we'd go for the 6T first, then the 3T, and the 5T in the third spot.
Oddly enough, the 3T has the sharpest 1080p footage where it's tied with the 5T, only the 5T's is noisier than the 3T's. The 6T's 1080p clip has a strong overprocessed look, though it is the cleanest, and evidently superior in dynamic range.
Walking around the park to test how the stabilization works revealed a flaw of the 3T's video recording, which we couldn't quite spot from the balcony. That is, in addition to the lack of stabilization in 2160p. You see, with all the shaking going on, the 3T's video camera had trouble maintaining focus, resulting in even more jittery footage, and that's in both 4K (with no electronic stabilization) and 1080p (where the stabilization is at work).
Now, it could be due to the fact that the 3T is a two-year-old phone that's been used and abused during its lifetime, while the 5T unit hasn't seen all that much action, and the 6T is practically new.
The 5T and 6T's focus is just fine in this test and their stabilization works well in 4K, with some warping anomalies we spotted in transitions between panning and holding still. Panning doesn't happen entirely perfectly on either, so just pan more slowly and you should be fine.
In 1080p the 5T and 6T behave similarly, and that's some pretty decent behavior, though we still did spot some wobble. And it's a bit disappointing to observe the overall softness that's got to have been brought about by the stabilization.
So the 6T's camera is in most ways superior to those on the 5T and the 3T, what else is new. Establishing that wasn't really our goal here. Instead, we were looking to find out what you stand to gain if your trusty 2-year-old 3T is showing sings of age, or if you're the less patient type and you're doing yearly upgrades, so the 5T has to go. Or just observe how OnePlus' cameras have evolved.
Well, the 6T has improved dynamic range across the board - be it video or stills. The 3T in particular is lagging behind a lot, but the 5T isn't quite on par with the 6T either. Low light photos are also a significant step up on the 6T compared to the older generations. If you're coming from the 3T, the 6T will also give you portraits, but the 5T already has that, so there's not much to be gained there.
There are a few things that haven't changed and we still feel need addressing on upcoming OnePlus phones. The selfie camera could benefit from having autofocus, or at least a different focus distance - as it stands right now, you need to choose between sharp focus and wider coverage. Video recording has been refined, but we'd still appreciate the ability to turn off the stabilization when not needed.
There's more to the OnePlus 6T than the camera, of course, and you can check out our findings in the in-depth review. From an imaging perspective alone, the 6T could have delivered more improvements over previous models, but it's still a worthy upgrade nonetheless.