Re: dreamy Nikon lenses
----- Original Message ----- From: "Jack Brubaker" <email@example.com>
Sent: Monday, July 21, 2008 7:09 PM
Subject: Re: dreamy Nikon lenses
Thank you all for the comments on varying the elements. I haveOne way to test lenses for spherical is to make a obstructive stop, that is one which blocks the center of the lens but admits light in a ring around the periphery. One then measures the focus shift between the two to get some idea of the amount of spherical. One can make obstructive stops to measure zonal spherical by making a ring of various diameters. Usually, lenses are corrected on the axis (by definition there because there is no deviation of the light) and at the edge, the greatest deviation is usually at about 0.707 of the radius.
One lens which makes use of something like this kind of stop is the Rodenstock Imagon. It is a chromatically corrected meniscus, like several other soft focus lenses, but has a series of special stops. These are open in the center and have perforations around the periphery. The stops vary in the relative amount of the openings. The outer perforated ring allows a variable amount of light though the edge controlling the amount of spherical aberration but also provides some diffusion, or really multiple images. The result is not quite like any other soft focus lens and is the reason Imagons are still made.
While a stop in front of a lens is not exactly the same as one between the elements it will still serve for experiment. Stops can be made of heavy construction paper (Kraft paper) or thin cardboard of the sort cereal boxes are made of.
Los Angeles, CA, USA