As background, I have a very low budget setup, complete with using an iPhone 6 as my camera (with Filmic Pro as the software, so I can manually change things like exposure and focus). I made a teleprompter which has been working for me out of a cardboard box (coated on the inside with black tape) and an iPad (since I'm reading song lyrics, I don't need an operator). I used to be able to get video that was good enough quality for my purposes. I am shooting with Cowboy Studio umbrella lights in my garage.
My question is this: since adding the makeshift teleprompter, the video became darker and much grainier. I think it's because it's basically shooting through a black tunnel, and I have to crop the edges off. Within Filmic Pro, I have manual access to ISO and shutter speed, as well as a separate "Exposure Compensation" which seems to change the previous two values. How can I remove the darkness and graininess? I've tried increasing the ISO or decreasing the shutter speed, but my video ended up looking like badly done stop-motion animation. Did I go too far? Do I need to change the lighting setup?