Mirror Lock-Up - What is MLU and How & When to Use It

Mirror Lock-Up - What is MLU and How & When to Use It
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Most conventional SLR (single-lens reflex) cameras with interchangeable lenses use a mirror to project light coming from the lens to the viewfinder. Currently all Canon, Nikon, and Sony cameras work this way. The exception comes from the Micro 4/3’s cameras which do not use a mirror. Amateur and professional photographers use these SLRs because they enjoy using the viewfinder and seeing the actual image as the camera sees it in real time. Using an LCD causes a delay and can be difficult to view (like being outside on a bright sunny day). In order for a photo to be taken, the mirror is flipped up, the shutter is opened for a certain period of time (equivalent to your shutter speed), the sensor is exposed to the incoming light, the shutter closes, and the mirror returns back into place creating that signature “clacking” sound each time an image is made.

How It Works

When an image is taken, the process described above with the addition of the aperture being set in the lens, is performed in an instant. The swift motion of the mirror flipping up to reveal the sensor introduces slight vibrations in the camera as it is “slapped” upwards. These vibrations can induce motion blur into your images. The blur is noticeable when zoomed in at 100% to the picture nd compared side by side with a photo taken with and without mirror lock-up (MLU). It’s a slight difference, but it is noticeable.

MLU solves this issue by flipping up the mirror ahead of time so that any lingering vibrations from the mirror slap are long gone. You are then free to take as many pictures as you want before flipping the mirror back down. Each subsequent image you take after enabling MLU, will only open and close the shutter and will not flip the mirror back down, resulting in a quieter camera noise.

Using MLU

MLU is a feature on most cameras, although you may have to go digging in the menus to find it. It is sometimes incorporated into the 2 or 10 second timer delay. After MLU is engaged and the shutter is pressed, the mirror is flipped up out of the way and the viewfinder goes dark because it is no longer receiving light. To take the photo, press the shutter release again.

When using MLU, place your camera on a tripod, compose your shot, enable MLU, press the shutter button to take an image (or optionally, use a remote shutter release cable to further prevent any camera shake), and then check your photo on the LCD, does it still look the same as before MLU was enabled? If not, then either the subject or your tripod has moved.

When To Use MLU to Get Motion Blur-Free Shots?

  • Landscapes
  • Astrophotography (lunar moon photography included)
  • Using a long telephoto or macro lens on a static (stationary) subject
  • Using slow shutter speeds
  • When you have a tripod and have time to set up the shot

Photo by: mikebaird

Moon taken with a 600mm + 2X TC (1200mm lens) with MLU enabled through live view (LV).

When Not To Use MLU?

In 99% of situations, you will not need to use MLU. It will slow you down and can be a hassle to enable on some cameras. It obviously shouldn’t be used if you do not have a tripod and are shooting a moving object (including portraits). I mainly only use it for astrophotography or low-light situations. You would be amazed though how far a tripod and a remote shutter release can go towards creating sharp images. Now that you know what MLU is why not give it a try the next time you are out shooting in a situation it seems fit?