John Cantelo said:
Thank you, Henry for some very informed and interesting comments.
I'd be grateful if you could clear up a couple of points:-
"If you are planning to buy a cost-no-object binocular I would wait to see the new Zeiss FL Victories which are due out in May"
- is there any info on the web about this new design?
"The Abbe-Koening roof prism currently used by Zeiss in the Victories and some of the old Classics has inherently higher light transmission than the Schmitt-Pechan prism used by Leica, Swarovski, Nikon and everybody else"
- interesting, but if this design is so clearly better, why doesn't everyone use it?
"Why won't Nikon do the birding world a favor and redesign the 8x32SE as a waterproof reverse porro with screw down eyecups? That would be a binocular!"
- as a Nikon 8x32 SE user you don't have to convince me that it's a great instrument. I'd agree re: redesigned eyecups and waterproofing (though mine have never leaked) plus I'd prefer a barrel style focus wheel. But reverse porro? Why? The current design is the most comfortable porro design I've ever used. What advantage would a reverse porro have?
John
John, Excellent questions. I’ll be happy to climb on the soap box and try to answer them. I guess it’s obvious I’m an iconoclast about some of this, and an obsessive to boot
(my wife wishes binoculars had never been invented), so bear with me. Maybe I can stimulate some discussion.
I think the Abbe-Koenig prism has been used so seldom because it has one serious disadvantage in binoculars. It’s a long narrow prism configuration, with a light path that doesn’t reduce the length between the objective and the eyepiece very much compared to just a hollow tube, Some of the old Hensoldts using AK prisms look to be a foot long. Porro and Pechan prisms have light paths that absorb more length, so binoculars using them will always be shorter than those using AK, if the focal length of the objectives are equal. You’ve probably noticed that the Zeiss 7x42 Dialyt, which uses AK prisms, is an unusually long binocular for it’s aperture. I think it would be even longer if a typical F:4 objective had been used. To make it as short as it is, the focal ratio was probably dropped to some figure below F:4
At the other end of the size scale not even Zeiss uses AK prisms in really small binoculars, possibly because there just isn’t enough space between the objective and the eyepiece in such a short focal length binocular for an AK prism of the size required to handle the light cone.
In the current Victory Zeiss attempted to make a binocular using an AKprism just as short and compact as one using a Pechan. To do that the focal length of the objective had to drop even more than in the Dialyt,with the focal ratio probably approaching F:3, necessitating the use of a complex objective and eyepiece to try to achieve a reasonable degree of correction of aberrations (which get worse the lower the focal ratio). Most people seem to agree that the result, except for light transmission and contrast, was not a complete success.
On paper the new binocular looks like a contender. It’s addressing the weaknesses in the current design with a Fluorite objective, improved eyepiece and judging from the dimensions, an increase in focal length.
Now on to reverse porros, a sadly underutilized design in high end binoculars. So far the design has been confined mostly to small inexpensive bins, but it could work with 32mm objectives, or even larger with a more vertical offset between eyepiece and objective. The Bushnell 8x30 Nature View, The Bausch&Lomb 8X50 Elite, and the old Rollei 7x42 hint at the potential.
Reverse porro bins have the same advantages over traditional porros that roof prisms have. First of all, because of the close spacing of the objectives they have the same illusion of higher magnification compared to traditional porros that people like so much in roof prisms .
Second and more important for birders, the close spacing of the objectives allows for much better merging of the left and right fields at close distances. Try comparing the view through a traditional porro and a roof or reverse porro at 3 meters. The two fields in the traditional porro will show only a small area of overlap, a bit like the cliche movie version of a binocular view, two slightly overlapping circles. The roof and reverse porro will show much better merging, closer to a single circle, allowing both eyes to come closer to the ideal of looking straight through the center of the field. The effect gradually diminishes with distance so that at long range all three types show something close to a single circle.
Finally reverse porros have the same advantages over roof prisms that traditional porros have. Lower cost and all other things being equal, higher optical quality. If the optics of the Nikon 8x32 SE were placed in an ergonomically sucessfull reverse porro housing (something hand-friendly we haven’t seen yet in a reverse porro ) the result would be a binocular with the things we like about viewing through roof prisms combined with the advantages of porro prisms in the same binocular.