In my experience, the eyepieces that deliver the highest-contrast images all have few air/glass interfaces -- typically four. Note, that that is not to say that every eyepiece with just four air/glass interfaces gives high contrast. I believe the reasons are that polishing is not always perfect -- and imperfectly polished surfaces scatter light, and furthermore that less-than-perfectly applied coatings may have rough surfaces, which also scatter light.
It is also true that no low-reflection coating transmits 100 percent of incident light. With a lot of air/glass interfaces, the loss might be noticeable: Thus if the coatings transmit ninety-nine percent and reflect one percent, transmission through an eyepiece with ten coated air/glass interfaces would be only about 90 percent (actually, 0.99 to the tenth power, which is 0.904...). That is not much, but every photon that is reflected back toward the sky has an additional chance to cause problems by again being reflected back toward the field of view, now out of focus, by one of those coatings. Take the ten air/glass interface eyepiece: Depending on which surface it is that reflects a photon back toward the sky, that photon will have, on the average, about five more chances to get redirected again by a 99 percent coating. Thus about five percent of the ten percent of photons that are reflected back toward the sky will return to cause glare in the image. That is half a percent of the incoming photons, which doesn't sound like much, but if you are looking for small-scale low-contrast detail on a bright lunar or planetary surface, or trying to spot the faint companion of a bright double star, the extra scatter from the bright stuff in the field of view might make a difference.
One quick way to evaluate coating quality is to look straight on into the ends of an eyepiece with clean lenses. If you can see any lens surfaces, the coatings are second-rate or worse. Looking through a well-coated lens should be like looking into a space warp of some kind -- the light appears distorted but there is no visible cause. Thus if you encounter a Maksutov or Schmidt-Cassegrain corrector that is well coated, it will be invisible -- the secondary will appear to be suspended in space, as if by anti-gravity.)
Another possible problem might be poor blackening of the edges of the various eyepiece lenses -- those surfaces are generally ground, and can scatter light. The more surfaces, and the thicker the lenses, the more scattering.
The eyepieces that I have personally used that seem to have the best contrast -- the lowest scattered light -- are classic Brandons and Pentax SMC-ED orthoscopics, all of which have only four air/glass interfaces.
Your mileage may well vary, and I expect that there are fancy eyepieces (with more than four air/glass interfaces) that do well for contrast, I just don't know which ones they are, and there are a lot of eyepiece types that I have not tried.
Clear sky ...
Edited by Jay_Reynolds_Freeman, 05 April 2025 - 06:53 AM.