Here is an example from my Sigma FP shooting simultaneous JPEG+RAW stills with the JPEG on the top and the RAW on the bottom.
The only differences are the expected ones regarding colour, contrast, WB etc baked into the JPEG by whatever profile I had active in the camera at the time.
This next one is from yesterday on my LX100 shooting RAW+JPEG, again straight out of camera with the JPEG on the top and the RAW on the bottom.
With this one we again have the expected differences from the JPEG profile but obviously from the RAW you can see the large amount of optical correction that the camera is performing to remove the distortion etc of the lens for the JPEG.
From the point of view of operating the camera, this is irrelevant as it has happened before it hits the EVF or LCD so your framing view will be of the corrected image. In terms of the JPEG or if you are shooting video then, again, its an irrelevance as the corrections are baked in.
For RAW files though, the corrections are not applied but they are only a click away in your editing software so, again, its no big deal.
The problem would come though if you somehow managed to remove that lens and put it on a camera that doesn't have the correction built in.
Which brings us to using MFT lenses on a Pocket 4K.
I've just gone outside to do this quick and dirty illustration of a typical example with the Panasonic/Leica 12-40mm f2.8-4 on my Panasonic GX80 shooting RAW+JPEG and Pocket 4K shooting CDNG.
On top we have the JPEG from the GX80, then the RAW from the GX80 in the middle and finally the CDNG from the Pocket 4K on the bottom. (* I was shooting 16:9 stills on the GX80 with regard to the JPEG but the RAW file is still 4:3)
Ignore the framing and colour differences as it was just a quick handheld test but as you can see the in camera corrections have taken a large amount of the distortion visible in the RAW away when producing the JPEG.
However, this is not the case with the P4K frame and without the in built correction the distortion is there in all its glory as it was with the RAW file on the GX80.
Of course, you can perform the corrections yourself on the P4K footage when you are editing but its another step and will have to be done on every clip. There is also the penalty of the correction necessitating some degree of cropping in most cases so, unlike the cameras with built in correction, you will have to take this into account for your framing when shooting.
The degree to which correction is necessary will vary a lot from lens to lens and obviously wider angle lenses will need closer attention hence why I would recommend checking each particular lens on the P4K itself before buying to make sure you are comfortable with it.
I think the Voigtlander 17.5mm is quite popular on here with people using MFT cameras so if someone could do the same RAW+JPEG still on a GH5 etc then you'd have a fair idea of how it will behave uncorrected. I think a few people use it with the P4K too so that would obviously be even more indicative.
