A break in the rain allowed me to get out and try my new Alpa iPhone Viewfinder today (please excuse the poor iPhone pics below). Using this viewfinder solution presents an interesting mix of lo-tech Alpa body & lenses/shutters alongside the hi-tech MFDB & iPhone viewfinder application.
Overall I'm very pleased with the performance of the Alpa eFinder app although it does have a few foibles due to the iPhone. For instance, in vertical mode with the camera aligned over the lens you have to use the iPhone inverted and the app displays the labels upside down, although the main display rotates correctly to display the lens framelines - not sure if this is purely an app issue or a limitation with software buttons on the iPhone iOS libs.
Frameline accuracy seems pretty decent and much better than the optical viewfinder for me. It's very easy to have parallax between your eye and the VF center with the optical VF but the iPhone removes this error. I did find that the vertical framing wasn't exact but I need to test that further. It wasn't a problem in use though and may have been due to me tilting the camera anyway. You still have to guess with shifts/rise.
For lenses wider than 37mm you'll want a wide angle adapter for the iPhone so that you can get the full preview. The eFinder app supports a wide variety of wide angle multipliers e.g. 0.8x - 0.38x plus obviously native 1x.
Front view - note iPhone upside down in vertical alignment configuration:
Framing an image - here's what you work with:
Example of preview in Alpa Viewfinder app & captured image:
Typical view with histogram:
Horizontal view with camera over center alignment of the lens: