The EFV gives you a TTL, dynamic view of the image, including white
balance, exposure, and DOF (which unlike DOF preview is not so dark
as to be unusable).
The EFV finder still only gives an approximation of what the image would look like (just like an optical viewfinder) for the things you mention.
There is far more processing done to the actual image taken than what is done to the image displayed on the EFV. The exposure will be more accurate when the photo is taken, than for the preview.
Also, the image captured by the sensor will be different due to a number of things, such as being taken at the actual aperature, not a simulated one. The speed of the shutter will have an effect on the image as well. If I have a slow shutter speed, the EVF doesn't show me what part of the image will be blurred any more than an optical one.
Also, don't forget that the EVF itself has it's own colour/contrast characteristics, which will affect how the image looks.
The resolution of the EVF also causes problems as it has to map about a 9x9 pixel area sensor area to a 1 pixel display area. This means that a lot of fine detail is lost, and one can't view it. I have viewed objects that had fine detail which did not show up on the EVF, but were in the image itself, and were viewable through an optical viewfinder. Try seeing the fine veins in a leaf for example.
A number of the EVF's will switch to B&W mode in lower light levels, which is definitely not showing the correct colour of the image.
EVF finders also don't show the effect of using flash (fill in or otherwise) on the image either, nor can they help with white balance in this case either.
Bottom line is that the EVF finder, like the optical finder, only provides an indication of what the actual image will be, and that is where the photographers skill comes in play into being able to visualize what the final image might look like.
With some more resolution, a "high gain" mode and some internal
magnifier for focusing aid, the EVF will be completely superior to
an optical viewfinder in just about every respect. Already it is
superior in some respects.
To "match" the resolution of an optical viewfinder, the EVF's resolution would have to match that of the sensor, and that is a long way off. However, in most cases, an EVF that has 4 or 5 times the current resolution would likely be acceptable in most cases.
EVF finders also have problems showing scenes where there is a lot of dynamic range, hilight areas become washed out and all shadow details are lost.
There is one aspect of an optical based viewfinder that an EVF will never be able to come close to matching and that is it's low power consumption. A lot of power is consumed in running the sensor, processing the data, and running the EVF finder. An optical viewfinder requires no power itself, and the sensor/processors/etc can be powered down to a standby status.
Plus the EFV can provide a much better heads-up platform for
displaying camera settings.
Optical viewfinders can and do display camera settings. Admittingly optical viewfinders tend to display this information outside of the actual image, where as EFV finders display it on top of the image. Personally, I prefer the optical approach, but that is personal thing I guess. Also, some optical viewfinders do present appropriate information over the image, such as grid lines or indication of which autofocus sensors are being used.
Maybe I have a good memory, but quite of the information shown by the EFV I don't need to be reminded of. I can remember what compression level I have choosen for example, what exposure mode I am in, etc.
Also, I can remember taking excellent photos with SLR cameras where the only information in the finder was a exposure needle that one would align between two marking to get the "correct" exposure.
In the discussion of EVF vs optical finders, I do find three things not being mentioned that much.
First of all, the EVF is somewhat redundent as all the cameras have LCD displays which can display the same image/information as the EVF finder. I actually find them better than the EVF finder, possibly because there is no heavy magnification is needed to view them. The EVF's main advantage seems to be in cases where light conditions make viewing the LCD image difficult. Maybe a better approach would be to work on shading the LCD display via popup/out shade. Or maybe even a clip on "viewfinder" cup for the LCD.
Having both types of viewfinders, optical and electronic (LCD), seems to offer the best of both approaches in all most all cases. I choose whichever one is best for the given situation.
Secondly, a lot of the so called advantages of the EVF finder, really don't matter that much in real life situations. First of all, most of the time, one can take a test shot and review it on the LCD in far better detail than a live preview of the image. Secondly, setting such as white balance and so forth are set ahead of taking the photo and by means that don't use the EVF finder itself.
With my optical viewfinder camera, to set a custom white balance, I simply fill the frame with a white image under the lighting conditions and then click ok to lock that white balance setting in the camera. Using an EVF finder, I guess one could adjust the white balance setting while observing the colour change of the image, but I don't think it would be as accurate.
Finally, pro cameras seem to always be optical and EVF finders tend to show up on the low end of the SLR type cameras. If EVF finders were better than optical ones, wouldn't Canon and Nikon be using them on their DSLR cameras? especially their top end ones where price is not an issue.
Not only that but there may be additional degredation in the
