6.11 Mirror Lock-up and Live View

On occasion you may find it useful to operate the camera with its main mirror fixed in the up position.  Recall from section 2.1 that the mirror assembly in an SLR camera not only permits you to see the through-the-lens (TTL) image in the viewfinder, but also allows for traditional autofocusing and autoexposure methods.  With the main mirror in the raised position, the camera—and its operator—become effectively blind, until the exposure is taken and the mirror flips back into its down position.  Unfortunately, the shock waves from the mirror flipping between its up and down positions can sometimes cause enough camera vibration during exposure to result in a noticeable loss of sharpness in the resulting image.
    For this reason, SLR cameras have long offered a mirror lock-up (MLU) feature, which allows the photographer to separately trigger the flipping of the mirror and the opening of the shutter.  By flipping up the mirror and then waiting a few seconds for the resulting vibrations to die down before opening the shutter, sharper images can sometimes be obtained, especially for slow shutter speeds and/or long focal lengths (when optical leveraging becomes a factor—see section 3.5).  This obviously requires a tripod, and it’s also a good idea to use a remote shutter release (see section 4.4), to avoid introducing camera shake directly with your fingers.
    With the advent of digital imaging, mirrorless camera operation becomes possible, because the shutter can be kept open and the live image continuously streamed to the LCD on the back of the camera.  Now the actual capturing of a photo can be decoupled from the action of both the mirror and the shutter, since the camera can simply copy the currently displayed image to the memory card at any time.  This new feature has been called Live View, and is standard for most point-and-shoot cameras, and is offered as an optional mode on most DSLRs produced nowadays. 
    For bird photography, Live View is useful primarily when manually focusing—such as when using stacked teleconverters (which generally disables autofocus capability on most cameras).  Since the video feed shown on the camera’s LCD can usually be zoomed in (digitally), manual adjustments to focusing can be more exacting than through a tiny viewfinder.  Furthermore, since Live View requires the mirror to stay in the raised position, the benefits of reduced vibration due to mirror slap are also accrued.

    There are some problems with Live View, however.  First, by keeping the shutter open for seconds or minutes at a time, rather than hundredths of a second as usual, a significant amount of heat can be generated and built up by the imaging electronics, which, as you’ll recall from section 2.5, can result in increased noise levels in the captured images.  Secondly, the traditional “phase-based” autofocus method can’t be used during Live View, because the phase-based method requires that light be diverted from the main imaging path.  As a result, many of the early DSLRs with Live View didn’t provide any autofocus capability when using this feature.  Some newer models employ the contrast-based AF method (see section 2.6), which traditionally has been slower than phase-based autofocus, though it may have the potential to be more accurate.  Improvements to contrast-based AF methods may be expected in the years ahead.