ROMSPROD

Same situation here! (Optex + 12-35 + Gh4)
I've tested on this lenses: 14-42 kit, 12-35, FD 50 1.4 , FD 24 2.8, oly. 45 1.8
 
I think that modern lenses work best with this kind of anamorphic adaptor.
For me 12-35 as wide as possible (around 20mm for no vignette) and f8 or similar get good image. (best of all of my lenses by far)
My optex unit came with rear element fliped (reverse hack for close focusing) and terrible image quality.
After fitting and aligned correctly the image get very decent.
My advice is to put optex on the taking lens as close as possible , and with de 58mm threat direct to the 12-35 seems to hit the rear element of the optex again the front of the 12-35, so i'm using a 58-58 step ring (cheap UV filter without glass ?).
Need time for more test, but would be great if we could share our results @ROMSPROD
sorry for my english, i'm from Spain.

I had the Optex a couple of years ago and I recall it has to be stopped down to get sharp like all focus through adapters. Unfortunately it will still not become that sharp even at f5.6. But as GH4 is so sharp this is not so bad maybe? The Tamron diopter is quite good. Nearly as good as the 0.5 Tokina.

Hey ROMSPROD,

1) it depends
2) I like 35mm on the 5D, I have no idea of how that should be on the GH4.
3) 75.8% tall or 133% wide.
4) it depends.
here's a couple links to help you
About this kind of lens:
http://tferradans.com/blog/?p=7204
About diopters:
http://tferradans.com/blog/?p=7282
About workflow and post production
http://tferradans.com/blog/?p=7370
also, there's the Diopter Thread here in this forum that will probably solve your questions regarding these auxiliary lenses.