At the risk of opening the valve on the geek tank and being inundated with internet expertise, I'll raise a question that presented itself recently.
It was in a darkened theatre with some pretty exciting lighting on the stage. The occasion was a dance show and the organisers requested no flash photography. Being restricted to the maximum aperture on my mirrorless-camera lenses wasn't too bad as I had a close view of the stage. The f:1.8 and f:2.8 lenses let in enough light with a high ISO.
The real problem came with framing the image - the camera body was a Fujifilm X-E2 . It has a very bright and detailed electronic view finder that seems to see deeper into the gloom than I can. Fine to use but one's eye becomes used to the bright image and then when you take it off to change setting on the camera or lens in the dark...you can't see zip...the dark adaptation has entirely gone.
Using the LCD screen would have been wrong as the glow of it would have disturbed other patrons.
In the end, I resorted to peering over the top of the X-E2's hot shoe, pointing in the general direction, and hoping for the best. It all worked out, but left the impression that it should have been done differently.
Perhaps the optical view finder of the Fujifilm X-Pro 1 would be a better answer - sure, it has illuminated frame lines within the OVF but these are bound to be less overpowering than the full bright EVF screen. And there is always the EVF on call with a touch of a lever.
I suspect that the X-Pro 1 might be the best professional answer - sort of a bright horse for a dark race...