http://www.champchoice.com/store/Main.aspx?p=ItemDetailOptions&item=390
That indeed is a good adjustable iris, but mostly designed for pistol or revolver deliberate shooting where the eye is centered in the eye-glass lense.
If aiming a rifle, that aim is usually taken through the top-left 'zone' of the lens (10 o'clock), which is why the more modern "progressive" lenses are less than useless in aiding iron sight shooting. Can't see the sights, can't see the target, either. That area of a progressive lense is out of focus at all ranges and is not your prescription.
Due to my difficulty with accurate shooting using Progressive lenses, I switched to bi-focals for my last 2 pair of glasses and have my long distance prescription all over, except for the reading/computer focus area.
Yes- the sights are fuzzy, but I can actually see the front, usually, with long barreled guns and the target is clear and sharp with my vision corrected to 20/15 vision.
I can see getting a smoothbore before too awfully long a period of time, so's I can have a built-in excuse for missing- with targets over 30yards away, that is.