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Message: <blockquote data-quote="edz" data-source="post: 1379292" data-attributes="member: 44073">To support comments that coatings make a difference in color cast (and obviously total transmission), I'm posting two paragraphs here from an article by Roland Christen, noted optics expert and Owner Astro Physics.Comparative Coatings and Obstructionsby Roland ChristenThe test for a really good coating is to set it outside in the shade with the eye end up so that light from the sky can flood the optics. The other end should have a black end cap attached. Look straight down into the eyepiece to see how much of the light is coming back out at you. This is the light that is reflected back from the various coated surfaces, light that should have gone through and be absorbed by the black end cap. The darker the optics look, the more light gets through to your eye, and the less is reflected back or scattered. The really bad oculars literally glow with reflected light.Although all the eyepieces in this picture are "multicoated", they are not optimized. The different elements in a typical wide angle ocular are made from different index glass. For each index there exists an optimum coating design, one that will achieve the lowest reflectivity for that element. For optimum efficiency, each element must be coated with a different coating design according to its refractive index. What is often done is to coat all elements with the same formula. This results in some wavelengths getting attenuated more than others, usually in the blue end of the spectrum. These eyepieces will show a distinct yellowing of the image, as well as an overall lower transmission efficiency.  This can be seen in the second image where I have compared the 25mm Zeiss Ortho with the 3rd ocular. You can see a definite yellowing of the white wall behind the ocular, as well as a faint reflection of my head and sky behind me, which is completely missing in the Zeiss. <a href="http://geogdata.csun.edu/~voltaire/roland/coating.html" target="_blank">http://geogdata.csun.edu/~voltaire/roland/coating.html</a></blockquote>

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