When you press the shutter button on your digital SLR camera to take a photograph, the first thing that happens is the mirror flips up and out of the way to expose the digital image sensor.
Sometimes if you have a long telephoto lens connected to your cameras body, this mirror flip can cause a slight vibration. This vibration can result in a blurred photograph similar to the effects of camera shake. Many photographers choose to use a manual setting called mirror lockup to help avoid this problem.
How to know when you need to use mirror lockup?
When you use a long telephoto lens and notice photo blur (or loss of image sharpness) even after trying a sturdy tripod and remote release.
Is there any disadvantages to mirror lockup?
The main disadvantage to using mirror lockup is that you cannot look through the viewfinder when the mirror is locked. For this reason, mirror lockup is implemented most often with long shutter speeds. For example, when taking a photograph of a landscape at dawn.
How to set your DSLR camera to mirror lockup?
With Canon digital SLR cameras the mirror lockup is hidden in the Custom Function (C.Fn) setting. Once in custom function choose Mirror Lockup / Enable.
Note: These articles often refer to Canon or Nikon SLR cameras because those are the models I am personally familiar with. If you own a different model simply check your camera manual on how to set specific settings. For the most part, the basic workings are very similar if not the same, from one SLR model to another.
Does it really matter if your digital SLR camera doesn’t have this option?
No, it doesn’t matter as it’s not a function that is implemented often. Photo blur due to mirror vibration is usually so minute that it isn’t easily noticeable. To most photographers, their photographs simply may not look as sharp as normal and need to be sharpened in an editing program like Photoshop.