The Biogon has an vertical FOV equivalent to 16mm in the 135 format. So you could use the 15mm Voigtlander or the 16mm VF for the Contax and visualize a square crop. There is also the Fotoman VF which accepts custom masks. It was inexpensive when I got mine but it doesn't look like it's available anymore.
You would need some sort of cold shoe extension to retract the VF. Not too hard to do.
You would need some sort of cold shoe extension to retract the VF. Not too hard to do.
