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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.
Fig.
6.11.1 : Eagle nest at 1200mm. At enormous focal lengths (1200mm
in this case),
focusing manually through the viewfinder can be an exercise in
frustration. By enabling
“Live View” mode and
zooming in on the bird, it’s often possible to perform manual
focusing with much greater precision. (1/640 sec, f/10, ISO 1250,
1200mm, manual
mode, no flash)
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.
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