My SLR is equipped with the mirror lockup function. What does it do, and when would I employ such a feature?
Mirror lockup is used to reduce vibrations with longer exposures.
When the mirror folds up, the camera shakes for a bit. For short exposure times this doesn't matter, but for times of a few seconds it will cause motion blur.
By locking up the mirror before, the camera will be still for the exposure.
Mirror lockup can function in a couple of different ways, depending on the camera:
- Mirror lockup with self-timer - when you press the shutter release button, the mirror flaps up immediately. The image is taken after the timer interval has passed (typically somewhere between 2 and 10 seconds, often configurable in the camera).
- Mirror lockup without self-timer - when you press the shutter release button, the mirror flaps up, but the image is not exposed until you press the shutter release button again.
The use of mirror lockup is to separate the event of opening the mirror - which can create some vibrations in the camera - from the exposure. These vibration can cause some motion blur. The effect depends very much on the shutter speed, but tends to peak at shutter speeds slower than 1/15 of a second, but faster than a couple of seconds. When the shutter speed goes beyond a few seconds, the short time during which the mirror-flap-vibration occurs is so short in relation to the full exposure time that the effect becomes neglectible.
Of course, when using mirror lockup without the self timer, you should use some sort of remote release so that you don't need to touch the camera for pressing the shutter release button.
There is an article at Bob Atkins' website explaining it more in detail, with diagrams and all.
First of all, what you have is probably mirror pre-fire rather than a true mirror lockup. Only a few cameras have ever had a true mirror lockup, and I don't know of anything current that does.
With a true mirror lockup, you lock up the mirror, and it stays up until you unlock it. This can be used in the same cases as mirror pre-fire to reduce camera shake. It's also useful in a few other cases though. For example, Nikon once made a 6mm lens that required true mirror lockup -- the back of the lens stuck so far into the camera that the mirror couldn't swing by it. To use it, you had to lock up the mirror, then mount the lens. You left the mirror locked up the entire time the lens was mounted. This meant you couldn't use the viewfinder with the lens at all. Fortunately, for a 6mm lens, you don't really need it -- its field of view was something like 220 degrees, so everything in the general direction where you pointed it (and more besides) was in the picture. In fact, the lens kind of wrapped around the whole front of the camera, so you generally held it almost by your finger-tips at arms length (and aimed upward a bit to keep your feet out of the picture). The alternative was to mount it on the end of a stick to hold it away from your body.
As far as mirror prefire goes, it's worth noting that its primary use is really for exposures in the general vicinity of a tenth of a second up to around a second or two. For longer exposures, like 30 seconds, it makes little real difference because the vibrations damp fairly quickly so most of the exposure time is relatively vibration free.
Mirror lockup will move the mirror out of the way, pause (often around 2 seconds), and then acutate the shutter. This will reduce shake in the camera as a function of the mirror slap and can substantially help sharpness for very long lenses, macro work, and long exposures where the shake from the slap can become apparent in the result.
Mirror lockup is most useful when using a very long telephoto lens or when doing high magnification macro work. Because the field of view in these cases is limited to a very small angle, they are the two situations that are most likely to result in vibration caused by the mirror to be visible in the exposure. As others have mentioned, the camera should be tripod mounted and you should use a cable release, wireless remote, or self timer to prevent vibration caused by pressing the shutter button.
One application I use it for is taking pictures of the night sky. Even with shutter speeds as high as 1/125 to 1/250 sec for the moon I get sharper results locking up the mirror when using an effective focal length of 640mm. Canon Super Telephoto lenses include an IS mode that is designed to be used while tripod mounted that will compensate for mirror vibration.
In general, once exposure times are over 1-2 seconds mirror slap becomes much less of an issue because the vibration duration is a much lower percentage of the total exposure time. Note that the duration of the vibration will be affected by the sturdiness of the camera mount. A sturdy tripod will kill the vibration much faster than a less sturdy tripod.
But mirror lockup can also be useful for exposures longer than one second or so if there are bright light sources in an otherwise fairly dark frame. Stars in the night sky, streetlights over a dark street, etc. can leave squiggly light trails due to that one second or so of mirror vibration even on a 30 second or longer exposure.