So I tried the +4 and it’s exactly what I hoped. The angle of view is wide enough to capture the surrounding environment and the depth of field is shallow enough to look weird af. Like a totally sharp person surrounded by an unnaturally blurry world.
F number is focal length/aperture diameter. So if you make the focal length shorter but keep the aperture the same size the f number will go down.
So for me my focal length went from 180mm to 105mm but the aperture remained the same size so the f number went from f/5.6 to f/3.3
The way it looks is like the diopter is taking the exact same image from the 180mm f/5.6 lens and shrinking the whole thing down. So in a way the shrunken image has more dof than the unshrunken image but relative to an actual 105mm lens you would have to buy a very expensive wide aperture large image circle lens to get the same look and they don’t really exist.
It would be pointless to use wide-angle adapters on a large format camera, because it would narrow the image circle. So, it would be a limitation on movements.
That said, I have a Horseman attachment that does the opposite. It converts a 150mm lens to a 300mm lens. Versus the front, it attaches behind the lens.