When you take a picture with a traditional Digital SLR the mirror moves. This movement can cause a small amount of blur in your picture. The Mirror Lock Up feature stops the movement of the mirror and therefore removes any chances of blur caused by mirror movement.
Mirror lock-up is generally only found on more expensive DSLRs, but the amount of blur caused by mirror movement is so insignificant that this probably doesn't need to be high on your list of priorities unless you want to sell your pictures commercially and need to ensure they will be absolutely pin-sharp even when enlarged.
Techniques such as using a shutter release cable, remote control or self timer are often used to make sure the camera is not being touched when a picture is being taken. This helps to minimise any blur in your pictures. Even these methods have no impact on stopping the mirror from moving when the shot is taken.
If you are someone who is looking for the ultimate in picture sharpness, the blur caused by the movement of the mirror may be a cause of some concern. But before you decide to ditch your current Digital SLR or ensure your first Digital SLR has a mirror lock feature there are a number of factors to consider.
The first point to consider is how bad is the blur caused by the movement of the mirror? The fact it is well down the list of culprits when it comes to blur creeping into your photos. There are circumstances when mirror blur is more significant, but in most cases it is no more than a very minor irritation. To put things into perspective here is where mirror movement blur comes in the Digital SLR Causes of Picture Blur Table
The biggest amount of blur in your photos will come if the camera is unable to lock focus on your subject. There can be a number of reasons for this. These include not enough light in the scene, not clear enough what the subject is and too much movement occuring. When any of these happen and the camera is unable to lock focus any blur caused by movement of the mirror is completely insignificant.
After being unable to lock focus, the next biggest culprit is the movement caused by hand holding your camera. Any movement of the camera while the shutter is open results in far more blur than that caused by mirror movement.
Digital SLR Basics
Digital SLRs and Digital Cameras Key Differences
Digital SLR or Compact System Cameras
Digital SLR Handling
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Digital SLR Guide Author
This guide was written by Ian Younger
Even the blur produced by pressing the shutter button when taking a picture is of far greater significance than that produced by movement of the mirror.
Down below blur caused when the shutter button is pressed comes the blur caused by the movement of the mirror. Therefore until you sort out the problems caused by hand holding and pressing the shutter button there is little point in locking up the mirror every time you take a photo.
To get round those problems, place your camera on a tripod and use a cable release or the self timer.
Mirror movement blur is more or less non existent if you are using fast shutter speeds. Therefore it is only when you use slower shutter speeds that any even minor blur may be caused.
Extra long shutter speeds that occur when using a bulb setting may cause more significant blur from the movement of the mirror.
Two Digital SLR innovations may well mean the end of any blur caused by the movement of the mirror. The first one is the introduction of Live View. The second is Sony's introduction of a translucent mirror.
When you use live view the camera breaks the picture taking process is two. It moves the mirror out the way so that you can view the scene on your LCD screen. So when you use live view the problem magically disappears.
The translucent mirror introduced by Sony means that the mirror is fixed in place and does not move when a picture is taken.