henry link
Well-known member
Arthur and Leif,
Probably "field flatteners " is a misnomer for these things since the binoculars that use them still have some field curvature. A diagram in an old Fujinon FMT-SX brochure shows a reduction in "distortion" and edge "astigmatism". I think most of the edge softness that can't be corrected by refocusing is astigmatism. It turns star points near the edge into lines, parallel to the edge of the field on one side of focus, perpendicular on the other side and little crosses at best focus. That's what I see near the edge of the field in the Zeiss 8X42 FL.
As to distortion, I can't see much in the flat field binoculars I use, and it varies from some pincushion in a Pentax 10X50 PIF to very slight barrel in a Fujinon 8X30 FMT-SX. It's true I haven't noticed any flat field binoculars with AFOV's above 65 degrees, but it seems that almost all "wide field" binoculars now have fields that fall between 60 and 65 degrees. Binoculars with 70 degree fields like the Nikon EII and Swift Audubon (and Arthur's beloved Leitz Binuxit) have unfortunately become rarities.
Henry
Probably "field flatteners " is a misnomer for these things since the binoculars that use them still have some field curvature. A diagram in an old Fujinon FMT-SX brochure shows a reduction in "distortion" and edge "astigmatism". I think most of the edge softness that can't be corrected by refocusing is astigmatism. It turns star points near the edge into lines, parallel to the edge of the field on one side of focus, perpendicular on the other side and little crosses at best focus. That's what I see near the edge of the field in the Zeiss 8X42 FL.
As to distortion, I can't see much in the flat field binoculars I use, and it varies from some pincushion in a Pentax 10X50 PIF to very slight barrel in a Fujinon 8X30 FMT-SX. It's true I haven't noticed any flat field binoculars with AFOV's above 65 degrees, but it seems that almost all "wide field" binoculars now have fields that fall between 60 and 65 degrees. Binoculars with 70 degree fields like the Nikon EII and Swift Audubon (and Arthur's beloved Leitz Binuxit) have unfortunately become rarities.
Henry