Viewfinder magnifier on the Sony A7 / A7r / A7s?
I wanted to write a quick post about this. I tried one of my custom viewfinder enlargers on the A7s. It does work, but unfortunately the sensor in the eye piece does not get enough space and the screen won't automatically switch on when you move the camera away from your face.
It does enlarge the viewfinder, and it does add more comfort to shooting, but unfortunately you would have to manually switch between the viewfinder and screen. As expected you need to bring your eye as close as it will go to the viewfinder to avoid vignetting, just like when it is attached to a DSLR.
The connection of the eyecup is different to other manufacturers, although I did manage to cut down another one to fit, carefully with a knife.