mirror lock up

Mainly used for accessing the cameras sensor for cleaning, it should be accessible via the menus within the camera.
 
Mirror lockup locks up the mirror out of the way, before the shutter opens. This means that if you are shooting long exposures, the motion of the mirror won't affect the shot. I use it for all my astro imaging. Not sure how you set it on the 50d, but it's buried in the menus on the 450d. I use it with the self timer so that the mirror lockup and exposure all take place with a single shutter remote press, otherwise, the first shutter press locks up the mirror, the second fires the shutter.
 
In the menu go to Custom Function 3, then 6 and enable mirror lock up. As well as giving access for sensor cleaning, it reduces vibration because the mirror does not fly up and drop back during the shot
 
You lock the mirror up through the menus. The benefit of it is that when you tripod mount your camera you can compose your shot, lock the mirror up then use a remote trigger or the self timer to activate the shutter and it reduces the amount of shake in the camera.

When the mirror is locked up then you won't be able to see anything through the viewfinder which is why you need to compose before doing it.

You can also use mirror lock up when cleaning your sensor.
 
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