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Message: <blockquote data-quote="perseid28" data-source="post: 3310846" data-attributes="member: 104041">Pages 6 and 8 of the attachment on Post #1 show the effect that we're seeing: <a href="http://www.birdforum.net/showthread.php?t=150406" target="_blank">http://www.birdforum.net/showthread.php?t=150406</a>. I was having a hard time sorting through all the misinformation online concerning phase correction in roof prisms--it's probably because the effect is so complicated that it took until the 1980s for Zeiss to figure out a way to cure the issue. But apparently it has to do with light interfering with itself at one of the roof prism edges and causing the odd diffraction effect if left untreated.From sifting through the information yesterday, this is my understanding (may someone who knows more please correct me): in roof prisms, light can take two separate (but equally "legitimate") paths through the prisms. The problem is that the two paths are not equal lengths and so the light is out of phase when the paths recombine and reach the observer. Since they are out of phase, the light waves can have destructive interference (which causes a dimming of the image) and also constructive interference (which causes a loss of contrast). By applying a coating in a certain place on the prism, one path is made slightly longer so that the phases are once again aligned. Porro prisms are not affected by this because all light takes the same path.It seems that light that is polarized in a certain direction takes one path, while light polarized in a different direction will take the other path. The reason this is important is that it enables a person to do a simple test to see whether the binoculars have phase coatings. On a sunny day, look through the binoculars at the sky about 90 degrees away from the sun, and hold a polarizing filter in front of the binoculars. Twist the filter, and if you see the whole sky brightening and darkening the binoculars have phase coating. If you only see half of the field of view behaving differently from the other (darkening while the other is getting lighter or staying the same, for example), then the binoculars are not phase coated.Apparently this was a hot topic 5-10 years ago, and then folks moved on to worrying about other things <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" /></blockquote>

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