In article , Carlos E.R.
wrote:

I never understood well why a digital camera needed a mechanical mirror.

the same reason film cameras did, so that the viewfinder has an optical
path through the lens.

while digital viewfinders are much better than they used to be, they
are still not as good as a ttl optical viewfinder, particularly with
sports and low light. there is also no battery drain with an optical
viewfinder and focusing is faster.

And the digital display is actually seeing what the "film" is seeing. It
might in fact apply the digital processing that the final photo is going
to get so that the photographer can best decide on settings and timing.

except that a digital display is delayed versus pure optical.

it takes time to read the data off the sensor, process it and send it
to the display.

the latency is shorter than it used to be and won't matter for still
life, but *will* matter for sports or other action photography as well
as very low light.

The point of the SLR was that the photographer would see the same as the
film was going to see. Well, the digital display is one step further on
that road.

it's on a different road, with different tradeoffs.

And as Tims points out, there can be a display inside the eyepiece instead.

there can, but it will never be as good as pure optical, at least not
until the laws of physics are overturned, which isn't going to happen
any time soon.

for example, try using a digital viewfinder in extremely low light.
either it blacks out because the light level is too low, the frame rate
drops to compensate or it amplifies what it can 'see' and the
viewfinder is too noisy to be of much use.

with an optical viewfinder, your eyes adjust.

there is also no battery drain with an optical viewfinder (or heat).