Despite my issues with the contrast-detect setup, it's Panasonic's best auto-focus system yet and delivers good results once you get used to it. It's also highly tunable in terms of speed and tracking behavior, so you can adjust to find an optimal setup. Hopefully, Panasonic will continue to improve it with firmware updates to make it more predictable.
Other aspects of the S1 are also top-notch. The 5-axis in-body stabilization (IBS) system is excellent, maybe the best on any mirrorless camera. With all the space it takes up, it gives the sensor a lot of room to move and compensate for shifting or rolling movements. I found I could take photos without any camera blur at all, even at shutter speeds down to 1/8th of a second. It also performs beautifully for video, but more on that in a second.
A big camera requires a big battery, and the 3,050 mAh cell on the S1 is chunky indeed. Even then, it's only rated to power the camera for 400 shots, compared to 700 on Sony's A7 III. In normal use, however, you should be able to double that, particularly if you engage the power saving settings on the camera. Video endurance is very good, letting you shoot for about two hours on a charge. However, you'll want to order extra batteries if you buy this camera.
The S1, as you'd expect, is an excellent mirrorless camera for video, but I do have a few bones to pick with Panasonic. First, let's talk about what's good: The 4K video at up to 30 fps is razor sharp, because the S1 reads the entire 24.2-megapixel sensor and downsamples it to 4K size, delivering crisp shots with zero aliasing and moire. Panasonic has really stepped up its sensor game, delivering rich and color-accurate video. On top of that, there are no recording length limits at most resolutions, other than at 1080p 180 fps.
Once again, the in-body and hybrid stabilization does an incredible job of smoothing out video, even while you're walking -- provided you're not too herky-jerky. If the optical stabilization won't do, Panasonic also offers digital E-stabilization, which uses a slightly smaller portion of the sensor to give it more room to shift in case of extreme movements. Being quite hefty, the S1 also has lots of inertia to resist minor wobbles. Put together, this is a great camera for handheld video.
I was able to shoot very sharp, color-rich video with the S1, and the extra depth of field available with the full-frame sensor opened up new artistic and practical possibilities for me. For instance, I use the older GH5s to record a lot of review videos, but when shooting interviews, it's difficult to get a soft-focused "bokeh" background to isolate the subject. With the S1, that's easy to do even with the f/4 24-105mm lens.
Shooting indoors in dim light, I found that the S1 delivered incredible results. Shots were usable with noise being well-controlled, even at ISOs as high as 51,200. I think it actually delivers results close to Sony's A7S II, the current low-light champ, and I don't say that lightly. Another plus: Rolling shutter is well controlled, so you won't get the rubbery video on fast camera movements that you see on Sony's A7 III, for instance.
As mentioned, the S1 has both headphone and microphone jacks, letting you monitor your interviews and other recordings accurately. Internally it will record 4K video at up to 30 fps in 4:2:0 10-bit mode, but only at a limited 72Mbps bit rate. At higher frame rates, up to 60 fps, you can capture 4:2:0 8 bit 4K video internally with an APS-C crop. 1080p video can be recorded at up to 180 fps. Right now, you can only output external 4K video with 8-bit 4:2:2 detail, and there's no V-Log option to give filmmakers improved dynamic range.
The S1's video recording modes compare favorably to other full-frame models (for now). However, I feel that Panasonic has made a mistake here with the S1. Video recording falls well short of its own GH5s, which does have V-Log and can record 4K 10-bit 4:2:2 30 fps video internally, and output 4K 60 fps 10-bit 4:2:2 video.
Panasonic will soon give shooters the same options on the S1, namely 4:2:2 10-bit 4K 30p internal video recording, 4:2:2 10-bit 4K 60p HDMI output, and full V-Log recording. Unfortunately, it will charge you an as-yet unknown price for that update, on top of the $2,500 you already paid for the camera. That seems an odd choice considering that Nikon will soon unlock 12-bit RAW recording -- an arguably better feature -- for the Z6 and Z7 via a free firmware update.
Another issue with video: Any 4K shot at speeds faster than 30 fps is cropped to APS-C size. On top of that, in all high-speed shooting modes, there's no option for manual exposure -- only automatic. That can be a problem in changing light conditions, as you'll notice distinctive switches in the image if the camera changes the aperture. That can be especially problematic during tracking shots, where high frame rates are often used and lighting conditions can change.
The video autofocus system works as well or maybe a bit better than the one on the GH5/GH5s. As with photos, the tracking system is particularly good at keeping your subject's eyes and face in focus. However, the contrast-detect system does tend to hunt, so your subject will occasionally go in and out of focus. This is particularly noticeable on out-of-focus backgrounds, which sometimes "pulse" in and out of focus, ruining the shot. After I fine-tuned the settings, the problem largely disappeared, but it still cropped up from time to time.
If you'd rather do manual focus, as many videographers prefer, Panasonic's system uses a "focus by wire" electronic system. However, they've hit on a good idea with it. Some lenses will have a linear response thanks to a focus clutch and distance scale. Best of all, you can define the amount of rotation required to go from close to infinity focus, letting you set the precision level you prefer.