Following are a couple of examples of how I block out the surroundings with the V C. My main image is at 3 X 2 RATIO.
My circle is LOOKING THROUGH the PEEP HOLE and how I use it - LIKE a TROMBONE SLIDER - in and out - to TARGET my SUBJECT LOCATION
This gives my 2 images in one tool - narrow part and full composition - at the same time. Best little tool I have found - 3 1/2 inches - outer frame, up to 3 inch SQUARE opening in the COPOSITION VIEWING PART.
WWW.COLORWHEEL.COM should have some info on this tool.