In all of the research I have done the only optical loupe with a diopter adjustment have found that by specs will cover the 3.5" screen of the BMPCC is the Hoodman hoodloupe with the Hood riser that converts the loupe to work with screens up to 4".

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There are several options for affixing it to the camera. I am not sure which direction I will go in as yet. One is a velcro neoprene sleeve which could be modified to go around the lens housing on the BMPCC by cutting a simple curve into the velcro straps.

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The other option which is very versatile but I think may be a bit clunky is called the Hoodman Hoodcrane. It adjusts in all directions and has the ability to hinge up and out of the way to allow naked viewing of the viewfinder. I have a call into Hoodman to discuss what the options are and make sure it will work with the hood riser mounted as well and get their overall advice.

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But this should provide a solid viewfinder that fully covers the screen and allows a nice bracing point with the eye for added steadiness. By the way Blue Star makes a chamois cover that fits this loupes eyepiece. It is the Small Round eyepiece and will help with perspiration and potential fogging which pressed up against the eyepiece on hot days.