All ideas which involve something being between the sensor and the lens are impossible.
A flat piece of glass changes the optical path - Bolex came up against this on their reflex cameras. You need a specially designed lens (they call them RX lenses) to compensate.
The only advantage I see of an optical finder is that you can always see your frame, even if the system is off. I still double-take on EVF's when the AC has helpfully shut down the camera during some downtime.
It sounds stupid, but it is very nice to always be able to see.
IMHO - nonsense to all this optical finder twaddle - worry about the what's in front of the camera.
An SLR TTL viewfinder produces a visual resolution of approximately 1800x1200, has zero latency, uses no power, and costs ~$100.
While an electronic system can't ever reach those specs, it may not have to. I would suggest that a practical EVF for stills would be acceptable with a resolution of 1280x848, a maximum latency of 20ms in EV0 (very dark) conditions, a maximum latency of <5ms in >EV8 conditions, a power usage of ~2W (including sensor and processing) to not impact battery usage too severely, and a cost of ~$500.
As far as I can tell, your current system needs a roughly 5x improvement in latency, 10x improvement in power consumption, and 5x improvement in cost to meet those specs.
As for low-light performance, I can tell you that I can see better with my SLR through an f15 optical train than I can with my EVF through an f4 optical train. This is mostly a function of the sensor and larger sensors can close that gap, but the reason EVFs have the odd reputation of working better in low-light is because they can increase their integration times. The Mallincam is famous for this in the astro community, but it uses integration times of up to 56 seconds. Obviously, this approach is not acceptable for stills, and I don't believe EVFs have any current advantage when low-latency is maintained as mentioned above, which will be required by still shooters used to using OVFs in dark conditions.
Finally, there's the issue of autofocus. A mirror enables phase-detection autofocus, which in my experience is at least two orders of magnitude faster, more sensitive to small subjects, and better at tracking than even the best contrast-detection autofocus. This may be the largest barrier to a mirrorless still camera intended to compete with SLRs, unless CD AF can be improved by a couple of orders of magnitude or a way can be found to do TTL PD AF without a mirror.
Hope that helps, and I wish you the best in your development efforts. I'll be interested to see what you come up with and what practical benefits it might bring to those of us that shoot stills in difficult conditions.
There's a BIG difference between pro still work and pro movie work. Even if the camera handles both very well, that doesn't mean its users will...
There is a LOT of potential in what you are doing here, and I wish you well... it is likely to be a long time before I personally consider replacing my Canons for stills, but if/when I do, I seriously doubt I'll even think about anything without an optical viewfinder.
Again, that's me... others will vary of course!
As for how many "primarily-for-professional-still-image-capture cameras" we have sold in the last year and change... just as many as we did motion cameras two years ago...
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