The mirror blocks the sensor during composition, so it is not possible to
use the screen for that.
Personally, I would never want to compose my shot, or time my shot via an
electronic viewfinder and screen.  There is no substitute for the true TTL
optical viewfinder, in my view.

Oh jeez...it's tard time again.

At least on all the cameras I've used, the screen is only for
reviewing.

-Joel

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It cannot be used for composing on nearly all the DSLRs. The mirror gets in
the way. Using an LCD for composition adds size, weight and complexity to an
already very complex camera system. Most DSLR users don't like the idea of
composing with the electronic screens -- for good reason.

1) The screens introduce a delay between composition shooting. If you wave
your hand in front of a typical point and shoot camera which uses the LCD for
composition, you will see that there is a significant delay between the time
your hand moves and when it shows up on the screen. That delay is the
difference between taking a picture of a bird or a bare twig.

2) The screens are fuzzy and often difficult to see in bright daylight. They
are useless for focusing or checking depth of field. They show too bright of
a picture in dim conditions.

3) The screens are a serious drain on the battery, which already has quite a
bit to do in the modern DSLR, what with operating the shutter, focusing
mechanism in the lens, image stabilization, metering, viewfinder lighting,
LCD operation, etc. Indeed, battery usage is such a problem that many pro
photographers turn the LCD display off even for review between shots.
Instead, they take a short break and turn the screen on for a quick review
for anything that needs to be re-shot.

4) LCD screens make holding the camera steady for hand-held photography more
difficult. You have to hold the camera out away from your body in order to
see the screen, which is much less stable and much more tiring, especially if
you are doing a lot of shooting. Mount a heavy lens and you would really have
a problem. The whole ergonomic design of an SLR camera is geared towards
holding the camera up to your eye. You hold the camera with one hand and the
lens with the other, using the lens hand to focus and zoom. Consumer point
and shoots have electric zoom motors, which are very slow. The whole point of
the DSLR is speed. We don't want to wait while an electric motor racks the
lens in and out. We want to twist that zoom ring and have our composition
right now, if not fifteen seconds ago. We DSLR users are not a patient lot.
Holding a camera with a big zoom out far enough to see the LCD while zooming
the lens, well, that just isn't going to work. We only have two hands, you
know.

You understand, DSLR users are not likely to be a bunch of hidebound
traditionalists who like doing things the old way. We use DSLRs, after all.
No, most of us would use an LCD screen to compose pictures if somehow the
drawbacks could be addressed.

Generally speaking DSLRs (and the D-80) don't do live preview.  The
reasons include:

1)  Mechanical shutter

2)  Mirror in the light path

3)  Video readout circuitry missing from the sensor chip (and part of
why the pixels are so much better form DSLRs is because of this, they
can use the extra space for other things).

This is a shame, because composing on the LCD is a huge improvement in
usability for many purposes, and can even contribute to longer hand-held
exposures without camera shake.

The ability to see the screen from an angle (greatly enhanced if the
screen tilts and swivels) makes low and high angle shots much easier to
take.

The boost to the light levels of the screen makes it easier to see
what's going on in very dim light situations.

It's of course pretty much hopeless for manual focusing.  And the design
would have to be such that pushing the shutter release captured the
actual moment being previewed on the screen at the time; if there were
temporal skew that would be hideous.

Oh, the hand-held exposure thing?  Put the strap around your neck,
adjust the length so that you can pull your elbows into your stomach,
grasp the sides of the camera, and push it out until the strap is tight
against your neck.  In my experience (on cameras that *do* support LCD
preview) I can hand-hold this way at lower shutter speeds than trying to
pull the camera up to my head.  The camera doesn't fit my head in any
position, so it's hard to hold really steady (and I've been doing
handheld available light work since 1969).

Yes, lots of people just wave the camera out in front of them at arm's
length, and that's generally pretty unstable, but what matters isn't
what bad things other photographers may do; what matters is what
pictures *you* can take with the camera and techniques.

According to Tony  <Tony001@rr.net.net>:

    Only for reviewing -- and for navigating the configuration
menus. and looking at the histogram for exposure evaluation after the
shot is taken.

    The reason is that the sensor is covered by both the shutter and
the mirror, so it *can't* see the image to compose with until the moment
of exposure.

    The benefits from this are that you don't have to close the
shutter, flush the sensor, and re-acquire to take a shot, so the delay
from shutter button press to capture of the image is a *lot* shorter
than the typical P&S digital camera.  Probably not a problem in a posed
shot, but for capturing images as things happen, it is a major benefit.

    And -- because the sensor is not running full time (as it is in
a P&S) -- the sensor is cooler and thus has less signal noise in the
image.

    And I find myself wondering (having used both types of digital
camera) exactly *why* you would want to compose with the rigidly mounted
display on a DSLR?  Yes, the hinged displays on high-level P&S cameras
can be nice for taking shots from otherwise awkward angles, but they are
usually not that convenient compared to the eye-level finder in the SLRs
-- digital and film.

    Certainly, holding the camera to your face as you compose and
take reduces shake in the image compared to holding the camera out at
arm's length to view the display for composition.

    Enjoy,
        DoN.