Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 326

Troubleshooting Guide
Movie shooting stops by itself.
If the card's writing speed is slow, movie shooting may stop
automatically. Use an SD Speed Class 6 "
find out the card's read/write speed, see the card manufacturer's Web
site, etc.
If the movie shooting time reaches 29 min. 59 sec., the movie shooting
will stop automatically.
The ISO speed cannot be set for movie shooting.
In shooting modes other than <a>, the ISO speed is set
automatically. In the <a> mode, you can freely set the ISO speed
(p.174).
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 (except
EF-S18-135mm f/3.5-5.6 IS STM).
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.
326
" or faster card. To