For iron-sight shooters, both firearms and air-guns, the simplest solution is the aperture as it exists in the EyePal. Apertures are used in cameras to manage Depth of Field (DoF). The smaller the aperture, the DoF produces an image wherethe foreground and background are in focus.
The larger the aperture, the foreground and background will be out of focus. Only the subject, say at 10 ft, will be in focus.
Check out the EyePal article in the April issue of Airgun Hobbyist magazine for more details.