There is something weird going on with lossless and or crop_rec recording. (At leats on 6d that is
)
With the higher resolution crop modes I always felt that I got less recording time then expected considering frames size and buffer memory.
But since we have lossless compression, frame size varies so I thought it was caused by the variation in compression.
But it still bothered me, so did some testing.
I have a crop mode setting which records in 5464x3072 at 5.6 FPS.
This resolution and frame rate setting is very heavy, lots of data, so the SD card doesn't have much time to get on speed and I think we're actually measuring used buffer size with this setting.
The buffer on the 6d is 255MB.
When I use plain 14 bit recording I get a video file of 235MB
When I use plain 10 bit recording I get a video file of 189MB
When I use lossless 14 bit recording I get a video file of 119MB
The only one I can explain with 255 buffer size in mind is plain 14 bit recording.The 10 bit and 14 bit lossless video files are much smaller then buffer size,
the 14 bit lossless is actually 2 times smaller then the buffer size 
How can it be ?The 14 bit file size of 232MB makes sense:
One frame in plain 14 bit is about 29MB, 255MB buffer divided by 29 = 8.8, since you want whole frames, this is rounded to 8 frames. 8 times 29MB = 232 MB which is exactly my video file size I get with normal 14 bit recording
The 10 bit file size of 189 is weird.
One 10 bit frame is about 21MB, 255MB buffer divided by 21 = 12 whole frames. 12 x 21MB = 252MB. But I get only 189MB
The 14 bit lossless is even more weird.
I extracted the MLV file with MLV dump to DNG files and let it recompress the frames. Frame size appears to be around 18MB. 255 buffer memory divided by 18MB = 14 whole frames and 252MB in video size. But I get only 119MB, that's half of what I expected
Another weird thing is that MLV_dump doesn't seem to extract all DNG's out of the file.
14 bit recording file of 232MB gives 6 DNG's of 29.4MB each and a audio file less then 0.5MB, which is one frame less then expected according to filesize.
10 bit recording file of 189MB gives 7 DNG's of 21MB each and a audio file less then 0.5MB, which are two frames less then expected according to filesize.
14 bit lossless file of 119MB gives 4 DNG's of 18.2MB each and a audio file less then 0.5MB, which are two frames less then expected according to filesize.
Conclusion:
With 14 bit lossless compression I get 4 frames, where I expect about 14 frames (according to buffer memory and frame size)
P.S. this is all done with te experimental 4K build off 22 June 2018 from the downloads page and a customised crop_rec module.
