Practically speaking, for spherical-ish surfaces at least, there shouldn't be any difference between prism obtained through decentration compared to prism obtained by surfacing. Of course, this assumes that the edge and center thickness of the lens remain constant.
Prism is produced when the front and back surfaces are at an angle with respect to each other at some point on the lens. Surfacing prism into a lens using a conventional generator and prism rings tips the back surface of the lens relative to the front surface.
The same effect can also be achieved by moving or decentering the back surface relative to the front surface. If you wedge a lever under a log, for instance, raising the lever so that it forms an angle to the ground, the log rolls away from the lever.
Decentering a lens with power works in much the same way. In this case, however, the angle formed by the steeper surface as it rolls forms a steeper angle than the flatter surface, resulting in a relative angle between the two (or prism).
Nevertheless, an illustration of the relationship between the front and back surfaces produced in any of these cases would reveal a similar configuration if they are all compared with the position of either the front or back surface held constant.
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