From: Mirror Lock Up?

It's used as a way to reduce camera shake when taking long exposures (however I've recently read a very interesting article that kinda debunked that process).

I thinkit's also used when cleaning the camera, it'll move the mirror out of the way to allow you to get in about the guts of the camera.

I may be wrong, I'm sure someone will have a better reason for it.

On the D3000 it's only used for sensor cleaning. Select Mirror Lock Up, remove the lens, press the shutter release and the mirror will raise and the shutter will open to expose the sensor. Turn the camera off to close the shutter & lower the mirror.

On the D3000 it's only used for sensor cleaning. Select Mirror Lock Up, remove the lens, press the shutter release and the mirror will raise and the shutter will open to expose the sensor. Turn the camera off to close the shutter & lower the mirror.

Ahhhhh......that makes perfect sense. I haven't been able to figure out how to use it, now I know why. Thanks greenwing