I'm now as confused as you are about the focus stacking
All that's documented in the manuals is 4k and 6k stacking from post-focus, so I wouldn't expect more than that. However, since Panasonic doesn't update manuals after initial release I've been unable to preclude the possibility of stacking from brackets as a later addition.
It’s not 20, but it’s not worse than if you fired off a burst with an 18mp sensor
Actually, that's not guaranteed. The 6k bit rate on the G9 is 150 Mb/s (200 Mb/s from the GH5). So, depending on the amount of similarity between frames, the number of compressed bits may be lower in 4k or 6k post-focus relative the amount of unique information per frame. (The BGH1 has 400 MB/s but
its manual has zero hits for post focus or focus stacking and it's in the video line, so I presume it lacks support.) This isn't a simple thing to quantify and most photographers aren't going to write the image analysis code to work out the details but there are a couple key things in practice.
The first is it's unclear if post-focus .mp4 gets converted to jpeg frames on camera with the inclusion SOOC jpeg processing components like i.Dynamic or i.Resolution but it seems they're most likely not involved. Off camera, these definitely don't apply. So there is a tradeoff between video fps for bracket acquisition speed and tone curve control which, in my experience, primarily affects exposure management for highlights. It can also be significant to noise. Panasonic doesn't document anything about how video mode settings affect post-focus but, at least on the G9, there's some evidence v-log is helpful to relaxing tone curve constraints.
The second is raw is an option in stills-based brackets but, so far as I know, raw video in post-focus isn't supported in m43 as its an S1 specific feature. This has tone curve and other processing implications somewhat beyond SOOC considerations. There are also differences in bit depth (8- or 10-bit video versus 12-bit raw) and (usually) sampling that aren't well documented but do have pixel peeping effects.
Since the value of capturing 18 MP 6k, 16 MP still, or 20 MP still frames rather than 8.3 MP 4k frames is presumably in ensuring the larger frames pixel peep well enough to be worthwhile it's perhaps worth noting that 6k needs 234 Mb/s to have the same bits per pixel as 100 Mb/s 4k. So far as I know, no one's ever done a comparison of stacks from 4k and 6k post-focus, well controlled or otherwise, but the mathematics don't put me in a hurry to get a 6k capable body.
Does that mean that it uses successive video frames from a video to stack into a still image?
As
@11GTCS said, the basic answer is yes. The more complex answer is video and still photography have different assumptions about the most desirable handling of rolling shutter artifacts. Stacking from post-focus is somewhere in the middle due to the translation from video frames to a still image. Since there's usually an effort to minimize motion during (auto)focus bracketing it tends not to matter but my experience is the effects can be significant in macro situations like flowers with some wind motion (and become more pronounced with camera vibration at photomacrographic magnifications).
(a focus-stacked image using an E-M5.3 - i.e. 20MP - is somewhat cropped too)
In practice it's nearly always the case alignment of stacked images results in cropping. The extent of the crop is controlled primarily by the bracketing method and lenses used but there is also some software contribution. (It's easier just to plan for the crop than to arrange near-ideal telecentricity or rear standard bracketing.)
Now that I'm thinking about it, maybe it's because 6K and 8K are NEW, so burst mode is a thing of the past that will go away once 6K and 8K become common on cameras.
Panasonic cut SH burst from the G95 but I suspect that's just feature crippling. Sony's flyers indicate the lack of 30-60 fps still burst isn't a sensor limitation so it's presumably due to some combination of processing engine and card write speed limitations in the direction
@DanS mentioned.
All of the Panasonic 4k cameras can use 4K to focus stack.
Definitely not the G7 and I think probably not the GX8 or GX80. All the later ones (including the GX800), yes.
Wrong gender there.
