Designed for the Canon 400/5.6-Metabones-E-M1 combo.

Mark II worked great, but it was flimsy and got broken or bent or both nearly every time out, so I had to do some re-thinking. I opted for a magnetic mount, that is, the base is the same as the last one, but I filed off the holder and glued a steel post on in its place. Then I took a small magnet from a bits holder (3mm thick, 6mm diameter) and glued it to a more stable frame. The tongue and groove insures that it stays aligned when in place. Now if it gets knocked, it just pops out of its holder and either sticks to the side or is held by the retaining string. There is a small piece of shrink tube around the post to act as a sort of socket for the frame.
The size is a little different also, that is about 13x17mm, which corresponds pretty close to the angle of view with the 400 on the E-M1. It is adjusted to about 25-30 meters, that is, anything closer I have to frame slightly below the cross hairs, and anything further away, slightly above. It is held in place with two Velcro straps, so it can easily be adjusted laterally. For the pictures I pulled the black cover back so you can see the underlying construction.
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This is the eye sight part that fits into the flash shoe:
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The finder together with the Metabones adapter has ment a HUGE improvement in my BiF keeper rate!