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Message: <blockquote data-quote="OPTIC_NUT" data-source="post: 3367567" data-attributes="member: 121951">"HOW DID YOU DETERMINE THE DESIGN AND NUMBER OF ELEMENTS IN THE ACULON EYEPIECE?"By taking it apart.  It's not that exotic...it has a molded plastic field lens  just like the aspherics you can buy for telescopes now.  BTW: telescope EP makers also sell 5 and 6 element EPs with asperical content in there....molded acrylic clad to glass...clever.   And.....inexpensive now.It also has a much higher magnification alone that thin-lens formulas predict for its tested fl.Do you think there are more elements?Given its low eye relief, that would be a major design blunder by Nikon.It's just on par for a 3-element pseudo-Kellner.There's nothing new."WHY DO YOU THINK THE RANGEMASTER EYEPIECE USED AN ASPHERICAL SURFACE? I HAVE DISASSEMBLED A RANGEMASTER EYEPIECE AND FOUND WHAT LOOKED TO ME LIKE A CONVENTIONAL 5-ELEMENT ERFLE."By the shape of reflections off a surface, and by the peculiar field effectsnear the edge compared to other Erfle forward doublets.An Erfle basically has a Barlow section up front. As that moves, you get different trade-offs.Its spacings are not carved on stone tablets, andneither are its field properties.And:  aspherical optics are almost always one surface  in the field-conditioning section that entrains the curvature  of the incoming light to make life easier for the other elements."WHAT “ADVANCED” DESIGN DID THE 7x35 ZEPHYR USE AND HOW DID YOU DETERMINE IT?"Taking it apart to clean it.   More than 3 elements, or 3 elements with anunexpected field width/quality and a field lens that makes handmagnifications of 4x, when the spherical formula and a spherical sectionmagnifier out put out 2.5x (for example).  I carry the example around.You can buy aspherical   magnifiers, by the way.  B&L, Eschenbach.The B&L isn't as thin and even as this beauty from old 7x50s.The Eschenbach is good."WHAT IS A “PRECISION FIELD” "That just means an ordinary ~50-55 degree apparent field made clearto the very edge and very flat by, basically, field-stopping and adjustingthe spacing of elements of a 4 or 5 element eyepiece."AND A “STRETCHED-ERFLE TYPE”? I’VE NEVER SEEN THESE TERMS EXCEPT IN YOUR POSTS."If you like I can call it : "an erfle that has the forward doublet set at a greater distance than it would be for wide-field use"Does that sound better?I've seen it on in many excellent brands of the"Featherweight Family" or the finer 70-seg-apparent designs.   Tower is the best I've seen with their higher models, with a Long-nosed Erfle.Oops, sorry:""an erfle that has the forward doublet set at a greater distance than it would be for wide-field use""There seems to be some assumption on your partthat eyepieces are produced in singular set ways with standard pieces, or not at all.   Historically, that hasn't really been true,and nowadays, many binoculars have eyepiece designsthat defy and easy placement in the old styles.  Just lookat almost any contemporary Zeiss.   There are things not documented that were done.I learned aspherical grinding of microwave ferrites fromAmpex, a Japanese company.  I find that, and the lens in my wallet,amusing whenever some say they were never made in glass,and never for cheaper binoculars.  There is more out there than the documentation.  For camera optics....they are very well covered  (and riddled with aspericals, and 7-layer beamcoatings 7 yrs before Zeiss). For binoculars?.....scant coverage.</blockquote>

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