How and when to use mirror lock?

[csondagar]

I have 300D with hacked firmware which offers a custom function of mirror lock, amonst other functions. How and when is the mirror lock used? Thanks.

[cmM]

MLU is generally used when you are concerned about camera shake affecting the sharpness of your picture. Vibrations from the camera, however, only affect your shot under certain circumstances (certain focal lenghts, shutter speeds, etc..). If you shoot at a high shutter speed, then camera vibrations from the reflex mirror won't have time to affect your shot. If you take a 30 second exposure, again, the vibrations won't do anything as they don't "shake" your camera for too long.
I don't use it too often, but It's used a lot with shotting macro. A lot of landscape photographers use it as well even though it's not necesary a lot of times. I use it sometimes, not too often though.

[robertwgross]

Mirror lockup is generally useful over a narrow range of shutter speeds. For the average camera, maybe from 1/8th second to 1/2 second. On some cameras, it might be a stop longer or shorter.

---Bob Gross---