Im sure on here once i read what the optical no was for the leupold 50ft adapter. im thinking it was like .08 of an optic what ever that means. RAIDER

It looks flat to the human eye but it isn't. It's a very slight positive lens, that is, fatter in the middle than at the edge. It's got a focal length of 50 feet. Parallel rays of light entering the lens would be concentrated into a spot, 50 feet behind the lens. The dioptre number is the reciprocal of the focal length in metres. A lens with a focal length of 1 metre has a dioptre strength of 1. A lens with a focal length of 500 mm is known as 2 dioptres, being 1 divided by 0.5 (metres). The CFA focal length in metres is 50 ft divided by 3.281 to get it into metres, close to 15.24. The reciprocal of 15.24 (the 1/x button on the calculator) is 0.0656. How they manage to grind a lens to such precision defeats me, maybe that's why they are so expensive, they probably get a high proportion of rejects.