Canon EOS Rebel T5i Instruction Manual page 331

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.178).
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
331