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Message: <blockquote data-quote="ronh" data-source="post: 1322276" data-attributes="member: 55514">For you optics buffs, here is an interesting concept, due to the great physicist  R. A. Stokes. First proposed as a method for blatantly exposing CA in well corrected achromatic telescopes, I think it helps us understand some of the color fringing effects seen in binoculars. What follows is my rephrasing from H. Dennis Taylor's classic, "Adustment and Testing of Telescope Objectives".The achromatic objective produces a longitudinal secondary spectrum, with yellow green focused closer to, and red and blue farther from the lens. The user will choose a focus setting somewhere between the two, as the best compromise for multicolored light. At this chosen point, consider the unfocused light that lies below the optical axis. It contains yellow green from the upper half of the objective lens, which has already crossed the optical axis, and also red and blue from the lower half of the objective, which has not yet crossed. Although lacking blue-green and orange, these colors mix together to give the general impression of white light. So, there is fringing, but it is not very noticeable, because the light is not strongly colored.Now imagine what happens if the lower half of the objective is covered. The unfocused light below the chosen focus point is now all yellow green, and as a result, stands out glaringly, as color fringes. The fringe above the focal point is purely made of red and blue, so looks purple.Covering half the objective and observing the effect is "Stokes' Test", and is an extremely sensitive test for color error. It is said to reveal errors even in the finest APO objectives.Now begins my binocentric interpretation. Blocking half the exit pupil of a binocular, which is an image of the objective, is equivalent to blocking half of the objective. So, if the eyes are not well positioned, and the edge of the iris cuts off part of the exit pupil, color fringes will be seen. Stokes test shows this form of lateral CA and longitudinal CA to have the same origin, so a reduction in longitudinal CA, by means of Fluoride glass objectives, should also reduce the view's sensitivity to eye position.The way in which the part of the objective viewed through becomes skewed off center is a little different when the eye opening is smaller than the binocular's exit pupil (7x50 by daylight) or vice versa (8x20 at dusk). But in either case, if the eye is not centered, one is looking through an off-center part of the objective, and the principle of Stokes' Test applies.There is another source of color fringing, near the edge of the field, that is due purely to a color effect in the eyepiece. I say this, because I have seen it in reflecting telescopes, whose objectives have no color error. Jupiter at the edge of view in fast Newtonian with a Nagler eyepiece, for example, takes on a decidedly putrid appearance, chartruse on one side, and magenta on the other.I don't know how to separate these two sources of lateral CA, they must mix it up somewhat. But understanding Stokes' Test gives an explanation for the common appearance of near-centerfield color fringes, if the eyes are not carefully positioned, and the reduction in this effect with ED objectives. So, is this old hat for you guys? Did I explain it OK? Am I missing something? Did I get something hopelessly mixed up?Ron</blockquote>

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