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.
