This feature is availible on my camera, but I have no idea what it does and have seen it refered to many times. Can someone please explain to me what this does. All responses will be greatly appreciated.
In most slr cameras, when the mirror goes up at the time the shutter is fired it causes alot of shake and vibration. At low shutter speeds 1/8th to 1-2s it can cause soft blury photos, also with long tele lenses ( 600mm+ ) because your shooting with such a narrow DOF and field of view the shake can throw the subject out of focus or even out of frame! By locking up the mirror prior to taking the shot you allow time for the vibration to subside, it works best with the self timmer or with long telephoto lenses and a cable release.
When you press the shutter button to take a photo, the mirror in the camera flips up and then the shutter opens to take the photo. Telephoto lenses and long shutter speeds make even the slightest movement of the camera cause an unsharp image, including that little mirror slapping up. Mirror lockup enables you to let the mirror flip up and then enable a short delay to let the vibrations stop before the shutter opens. Different cameras do this in different ways. Unless you shoot a lot with long telephoto lenses and/or long exposures under exisiting light you won't use it very often, but its quite handy to have. Also, you should have the camera on a tripod when using mirror lockup, or it becomes totally pointless.
It is exactly what it sounds like. The mirror goes into the pre-exposure up position and stays there until you tell the camera to put it back into its normal resting position. Mirror lock-up is useful in eliminating one source of internal vibrations that can affect image sharpness at high image magnifications and slow (1/4-1/60 second shutter speeds). The vibrations that occur when the mirror goes up are referred to as "mirror slap" and can be bothersome when doing photomacrography or wildlife photography with long (>300 mm) lenses. Under these conditions, the very slight movement of the camera as it vibrates is magnified to the point that it can make the image appear to be slightly out of focus even though your focus is absolutely perfect and the camera is on a tripod. Any time you put your camera on a tripod, you should release the shutter with a remote release appropriate for your camera.
When you take a picture with your SLR, the mirror will flick upward and out of the way before the shutter open and expose the film. The mirror has some weight to it and when it is in action, it cause some vibration to the camera and vibration will make your picture less sharp. By locking the mirror prior to exposing the film will eliminate this mirror vibration and help you get sharper photos. Mirror lock up is most useful when you shutter speed is between 1/2 sec to about 1/60 sec. (That means most of the time.) However, when mirror lock up is engaged, you can't see anything in the viewfinder, it is best to use with your tripod. You will find it most useful if you are shooting something still and/or something close up.
There have been some interesting articles in past issues of PP&I showing comparisons between photos with and without mirror lock-up. The foregoing comments are all on the mark. Personally, I don't believe cameras with damping mechanisms on the mirror equal the sharpness of mirror lock-up.
BTW, it also seems to be a good idea to not only place the camera on a tripod, but also to provide some support for long lenses since any vibrations travel back and forth through both and can cause blurred images unless given sufficient time to damp out.
I put this in for information only and not as a biginning for debate.
Here is a site you may find interesting http://www.photo.net/nature/mlu -- it discusses mirror lock-up. And, you will notice this topic generates controversy. If you can find some articles by George Lepp, he goes into the practical end -- he among others have found the worst shutter speed for camera shake (vibration) occurs at 1/15 second.
This is going back over 20 years ago but if I remember correctly from a book on the Nikon F....
The Nikon F 35mm SLR was the first to have MLU.
The primary reasons for MLU was:
1. The reflex mirror can be locked in the up position out of the optical path for use with the Fisheye-Nikkor 6mm f/5.6 and the OP Fisheye-Nikkor 10mm f/5.6, whose rear elements protrude into the camera body and interfere with the movement of the mirror.
2. Locking up is also necessary for shooting with the Nikon Motor Drive at its top speed of 4 frames per second.
Using the MLU for vibration reduction was a bonus feature.