Without stepping on Henry's toes - all roof prism bins before phase-coating were inferior to good quality porro's...even if the porro's were 10 times less expensive.
I did a comparison between phase-coated roofs and roofs without phase coating some 15 years or so ago:
"I'm sure everyone here knows that roof prism binoculars *without phase-corrected prisms* are optically quite clearly inferior to those with phase-corrected prisms. The reasons underlying this problem have long been known. Albert Koenig and Horst Koehler, for instance, mentioned this effect in their book "Die Fernrohre und Entfernungsmesser" (3rd edition, 1959). An excellent explanation was published by Adolf Weyrauch and Bernd Doerband in 1988 in the "Deutsche Optikerzeitung".
I got my first pair of roof prism binoculars with phase-coated prisms, a pair of Zeiss 8x30B's, immediately after Zeiss started selling them, and I found the difference in direct comparisons to older Zeiss 8x30's quite marked. It was basically just as Weyrauch/Doerband had written in their paper - better resolution, slightly higher contrast, overall a "more pleasing image".
Last autumn I finally had a chance to do a more detailed comparison. We (a couple of fellow birders and I) got together for a weekend trip, and as there wasn't much about we had the time to do a detailed comparison of three different Zeiss 10x40B's. The first one was bought in 1979. It doesn't have T* coatings and the prisms are not phase-corrected. The second one was bought in 1981, with T* coatings but still without phase-corrected prisms. The third one was purchased in 1998, so it has both T* coatings and phase-corrected prisms. The interesting thing about this comparison was that all three binoculars were of the same make and had the same specifications, so all the differences observed were caused by the different coating technologies used. We compared the binoculars mounted on tripods, checking for their optical quality by looking at birds and a Zeiss standard resolution target in a variety of light conditions.
Perhaps the most interesting result initially was that the differences between the two old Zeiss 10x40B's weren't all that great. Sure, the T*-coated pair had slightly better contrast with cleaner colours and a slightly brighter image, but the difference was nothing to boast about. Even under difficult lighting conditions the difference wasn't that great. The resolution was exactly the same (as it should be), and the image of both was slightly fuzzy. This was most noticeable when checking the resolution targets. The comparison with the pair with phase-corrected prisms was almost a foregone conclusion after these results. And sure, it had much better contrast and cleaner colours, a brighter image and quite clearly a higher resolution than either of the two pairs without phase-corrected prisms. In fact, the difference was nothing short of amazing.
I believe this comparison puts some of the claims made about modern multicoatings into perspective. Modern multicoatings are nice, but they're not the most important thing to watch out for in roof prism binoculars. Based on this comparison I'd say the most important progress has not been the development of modern muticoatings, but rather the development of phase-correction coatings on the prisms.
One other interesting observation we made was this: After we'd done our comparisons I got my old Zeiss West 10x50 Porros (~1963) from the car. My friends had got bored with testing optics by that time, so we only did a quick comparison with the new Zeiss 10x40B's, and the results were pretty amazing. Sure, the 10x40's had better contrast and a brighter image, after all, the old 10x50's only have a simple single-layer coating, but the resolution of the old 10x50's was quite noticeably *better*. In fact, the difference was so pronounced that we couldn't help but wonder why Zeiss doesn't make these binoculars with a modern T*-coating anymore."
So, a modern Trinovid, based on the old Trinovid, but with modern coatings, will run circles around any old Trinovid without such coatings. It's as simple as that. I have access to a well-preserved Leica Trinovid 10x40 made in the early 1980s, and I wouldn't really want to use it for birding anymore. Just like I don't use my old Zeiss Dialyt 10x40 BGA anymore. Without phase-coating they're just not good enough.
Hermann
In Bill's book the paragraph below about the use of Nitrogen comes immediately before the one that I quoted in my post above.
