Canon EOS 6D Instruction Manual page 363

The manually set ISO speed changes when switching to movie shooting.
If you shoot a movie when [Maximum: 25600] is set with [ISO

speed range] and ISO speed is set to ISO 16000/20000/25600,
the ISO speed will switch to ISO 12800 (during movie shooting
with manual exposure). Even if you switch back to still photo
shooting, the ISO speed will not revert to the original setting.
If you shoot a movie when 1. L (ISO 50) or 2. H1 (ISO 51200)/H2

(ISO 102400) is set, the ISO speed setting will switch to 1. ISO
100 or 2. H (ISO 25600) respectively (during movie shooting with
manual exposure). Even if you switch back to still photo shooting,
the ISO speed will not revert to the original setting.
The exposure changes during movie shooting.
If you change the shutter speed or aperture during movie shooting,

the changes in the exposure may be recorded.
Zooming the lens during movie shooting can cause changes in the

exposure regardless of whether the lens' maximum aperture
changes or not. Changes in the exposure may be recorded as a
result.
The subject looks distorted during movie shooting.
If you move the camera to the left or right quickly (high-speed

panning) or shoot a moving subject, the image may look distorted.
The image flickers or horizontal stripes appear during movie shooting.
Flickering, horizontal stripes (noise), or irregular exposures can be

caused by fluorescent light, LED bulbs, or other light sources
during movie shooting. Also, changes in the exposure (brightness)
or color tone may also be recorded. In the <a> mode, a slow
shutter speed may solve the problem.
Troubleshooting Guide
363