Canon EOS-1 DX Mark III Advanced User's Manual page 301

Caution
Movie digital IS will not function when the lens's optical Image Stabilizer switch is
set to < >.
Although movies can also be recorded with Movie digital IS when [ ] is set, the
image area is further reduced.
With a lens whose focal length is longer than 800 mm, Movie digital IS will not
function.
Stabilization by Movie digital IS may be less effective at some movie recording
sizes.
The wider the angle of view (wide angle), the more effective the image stabilization
will be. The narrower the angle of view (telephoto), the less effective the image
stabilization will be.
When using a tripod, setting Movie digital IS to [Disable] is recommended.
Depending on the subject and shooting conditions, the subject may blur noticeably
(the subject momentarily looks out of focus) due to the effects of the Movie digital
IS.
When using a TS-E lens, fish-eye lens, or non-Canon lens, setting Movie digital IS
to [Disable] is recommended.
Since Movie digital IS magnifies the image, the image looks more grainy. Noise,
dots of light, etc. may also become noticeable.
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