Mirror Lockup: When to Use It

Rose at Brookside Gardens
(1/25sec, mirror lockup)
Under most circumstances, having your camera on a good tripod will allow you to take sharp pictures.  But when shooting with shutter speeds between 1/2sec and 1/60sec or so, the camera itself can cause a problem.  Your SLR camera is equipped with a mirror that allows you to see through the lens of your camera before you take your picture.  This mirror covers the sensor of your camera.  Whenever you trip your shutter, that mirror has to get out of the way of the sensor, so it slaps up against the top of your camera.  That action can cause vibrations in your camera.  For fast shutter speeds, the picture is taken before these vibrations can cause an issue.  For very slow shutter speeds, the camera is still long enough that these vibrations do not significantly effect the image.  But somewhere in the range of 1/2sec to 1/60sec (actual values will vary based on your camera, lens and tripod), these vibrations can cause some blurring of your image.  This is particularly a problem in closeup photography--for instance, if you're shooting pictures of flowers.

1/20sec exposure, mirror lockup
The solution to this problem is mirror lockup.  When mirror lockup is activated, you press the shutter button, and you'll hear the mirror slap up against the camera, and you will no longer be able to see your image in the viewfinder.  Wait a second for vibrations to die down, and then press the shutter button again to trip the shutter.  There are three things to be careful about here:
  1. On breezy days, the wind may move your subject between your first and second time pressing the shutter button.
  2. In changing light conditions, proper exposure may change--if you press the shutter the first time and see the sun come out before pressing it again, you'll likely need to take another picture.
  3. If you need mirror lockup activated, you probably need to be using a remote shutter release as well, since your finger can cause camera movement when pressing the shutter.
Note: Not all entry level SLR cameras have this feature, so check your camera manual to see if you do and how to turn it on.  Often this is a custom function you activate via the menu system. If you do not have this feature, exposure delay may serve the same purpose--this feature delays exposure by a second.  I know this works with the Nikon D90.  There's a simple way to see if this will work on your camera.  Take a picture with exposure delay on and look through the viewfinder.  If you can't see through the lens during that second delay, you have a good work around.  Another option is to shoot with live view on. When live view is on, the mirror doesn't slap up, so you won't have vibrations.