Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 196

Movie Shooting Cautions
Recording
If you use a card with a slow writing speed, a five-level
indicator may appear on the right of the screen during movie
shooting. It indicates how much data has not yet been
written to the card (remaining capacity of the internal buffer
memory). The slower the card, the faster the indicator will
climb upward. If the indicator becomes full, movie shooting
will stop automatically. If the card has a fast writing speed,
the indicator will either not appear or the level (if displayed)
will hardly go upward. First, shoot a few test movies to see if
the card can write fast enough.
If there is a very bright light source in the picture, the bright area may
appear black on the LCD monitor. In movies, the bright areas will be
recorded in almost the same way you see it on the LCD monitor.
Still Photo Shooting During Movie Shooting
Regarding the image quality of still photos, see "Image Quality" on page
165.
TV Connection
If you connect the camera to a TV set (p.258, 261) and shoot a movie,
the TV will not output any sound during the shooting. However, the
sound will be properly recorded.
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Indicator