Canon eos rebel T2i - Manual (Page 106)

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The camera’s mechanical shake caused by the reflex mirror action can
blur images taken with a super telephoto lens or close-up (macro) lens.
In such cases, mirror lockup is effective.
Mirror lockup is enabled by selecting the [7 Custom Functions
(C.Fn)] menu’s [Mirror lockup] and setting it to [1: Enable] (p.195).
1
Focus the subject, press the shutter button completely.
X The mirror will swing up.
2
Press the shutter button completely again.
X The picture is taken and the mirror goes back down.
Using the self-timer <l> with mirror lockup.
When you press the shutter button completely, the mirror locks up,
then the picture is taken 2 sec. later.
Remote control shooting.
Since you do not touch the camera when the picture is taken, remote
control shooting together with mirror lockup can further prevent
camera shake. With Remote Controller RC-6 set to a 2-sec. delay,
press the transmit button, the mirror will lockup and the picture will
be taken 2 sec. later.
Preventing Camera ShakeN
Shooting Tips
Do not point the camera toward the sun. The sun’s heat can damage the
camera’s internal components.
If you use the self-timer and mirror lockup in combination with a bulb exposure,
keep pressing the shutter button completely (self-timer delay time + bulb
exposure time). If you let go of the shutter button during the self-timer
countdown, there will be a shutter-release sound, but no picture will be taken.
Even if (Continuous shooting) has been set, single shooting will
take effect.
If 30 seconds elapse after the mirror has locked up, it will go back down
automatically. Pressing the shutter button completely again locks up the
mirror again.
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