BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="henry link" data-source="post: 1428197" data-attributes="member: 6806"><p>The FK designation might mean FK61 is a fluor-crown with some fluoride in the glass mix, but the Abbe number is only 82, so it doesn't have the same potential for color correction as pure CaF2 or FPL53 with Abbe numbers of 95. But, we are talking about 8x binoculars with f/4 objectives here, not highly corrected APO refractors. Astro-Physics has used different glass types (and Fluorite) in the past. The current scopes use FPL53, which I believe has only been available for about 8-9 years.</p></blockquote><p></p>