You're not missing anything, I just didn't explain myself well. I need to take two images. I mount the sensor vertically, shifted left, then shift right, blend the two images and crop to 54x54 (or whatever ratio). But it is hard for me to visualize the square through the standard viewfinder. Creating a square 54x54 mask on the ground glass will help me with composition and rough focus. Then mount the back, fine tune focus and shoot the two images.
Not like a real 6x6 experience. It is of course upside down and backward, unlike waste level that is just reversed. But at least I get to visualize the final composition without buying a whole new system.
Dave
Ah, that's more understandable.
I love square format. I often set the Olympus E-M1 or E-PL1 to 1:1 format (it just sets the cropped dimension on the raw file, but it allows me to visualize the square in the viewfinder or LCD. With other cameras I just aim for the center of the frame and apply a square crop in LR on import ... can always move it later.
With the E-M1, I can even have my waist level viewing back due to the tilt LCD, and given the 3:4 native format, I'm 'wasting' less of the capture with the square crop than I am with 2:3 format cameras. (E-M1 square = 12 Mpixel, E-PL1 square = 9 Mpixels).
But cropping FF or FourThirds formats produces significantly different FoV-DoF compared to 6x6 format. And then there are the Hasselblad's Zeiss lenses, which render differently compared to Panasonic, Olympus, Leica and Voigtländer lenses ... Something about the Hassy SWC's Biogon 38mm f/4.5 T* is not reproducible with any other lens I've tried.
G
Polaroid SX-70 shoots squares too!