I think that the interest of having an EVF also in a DSRL is
related to the possibility to compose the image with great care in
situations in which it is not so easy to see through the standard
optical viewfinder. For instance, i like to compose still life
images using the EVF of my small Canon S50 better than using the
optical viewfinder of my Contax Aria, although it is a good one.
Obviously when shooting in the street i prefere by far the optical
VF. Therefore, in my opinion, the possibility to have BOTH the EVF
and the optical VF would be wellcome.
I think you're confused about what "EVF" means. An "EVF" is an
electronic viewfinder that is otherwise similar to an optical
viewfinder, where you put your eye up to it to view the image.
The LCD display on the BACK of a camera is
not an EVF even when
it shows a live image.
I also do not understand why it is SO difficult. Clearly there is
the mirror, but the mirror could be lifted (as it has obviously to
be done when shooting) and the image directly obtained from the
sensor. I think this is the way Fuji S3 works, unfortunately with
many limitations.
Point & shoot cameras that offer a live LCD preview typically use
much less sophisticated autofocus and exposure systems than an SLR
camera, and this is largely due to differences in the optical path.
With an SLR, the sensors for the autofocus system or the exposure
system are typically located in the viewfinder prism. If you lift
the mirror so that the image sensor will be exposed, then the
optical viewfinder will no longer have an image, so the autofocus
and exposure systems won't work.
The only way around this is to use a beam splitter or
semi-transparent mirror instead of a reflex system, so that the
optical viewfinder and image sensor BOTH get some of the incoming
light. But this results in reduced viewfinder brightness and a
reduction in the amount of light hitting the image sensor. There
have been a few cameras that do this (i.e. Olympus E-10 & E-20) but
not everybody considers it a worthwhile tradeoff.
Furthermore, this sort of system means that the image sensor must
be exposed ALL the time, which is a problem if you're using a
mechanical shutter because now when the trigger is pressed, you've
got to close the shutter, then fire it for the exposure, then open
it again. You're doing twice the amount of work, which is going to
have a big impact on shutter lag.
Mike