Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 114

a: Manual Exposure
In [z2: Auto Lighting Optimizer], if the checkmark <X> for [Disable
during man expo] is removed, it can be set in the <a> mode (p.123).
When ISO Auto is set, you can press the <A> button to lock the ISO speed.
After recomposing the picture, you can see the exposure level difference
on the exposure level indicator (p.22, 23) compared to when you pressed
the <A> button.
I Using the Built-in Flash
To obtain a correct flash exposure, the flash output will be set
automatically (autoflash exposure) to match the manually-set aperture.
The shutter speed can be set from 1/200 sec. to 30 sec.
BULB: Bulb Exposures
During the bulb exposure, do not point the lens toward the sun. The
sun's heat can damage the camera's internal components.
Since bulb exposures produce more noise than usual, the image may
look a little grainy.
When [z3: Long exp. noise reduction] is set to [Auto] or [Enable],
noise generated by the long exposure can be reduced (p.125).
For bulb exposures, using a tripod and Remote Switch (sold separately,
p.304) is recommended.
You can also use a remote controller (sold separately, p.303) for bulb
exposures. When you press the remote controller's transmit button, the
bulb exposure will start immediately or 2 sec. later. Press the button
again to stop the bulb exposure.
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A bulb exposure keeps the shutter open
for as long as you hold down the shutter
button. It can be used to photograph
fireworks and other subjects requiring
long exposures.
In step 2 on the preceding page, turn the
<6> dial to the left to set <BULB>.
The elapsed exposure time will be
displayed on the LCD monitor.