School me on mirror lock up

[peteg1]

When do you use it, would I use it on landscapes/sunsets shots?

[Matthew Patrick]

Yes , you could use it for landscapes or anytime you want to minimize vibration caused by the mirror moving. I use it for copy work and the occasional landscape.

[blueM]

With tripod & cable release

[Wilt]

Past testing showed greatest vulnerability to mirror-induced shake sets in when the vibration is present during a significant portion of an open shutter. So in the range of about 1/8 to 3 sec, IIRC. Longer shutter open time, the vibration is during a small percentage of the total exposure time. Shorter shutter open time, the shutter minimizes the mirror vibration. That testing, BTW, was done in the days of conventional horizontal movement cloth focal plane shutters. How the vertical travel shutters of the dSLR would affect results is unknown!

[JeffreyG]

It can make a difference up to 1/60.