Sometimes they appear on the internet on sites like ..bay and on yard sales. The classic Japanese binoculars of the porro 7x50 type. Exactly the type I grew up with when my father once owned a pair of.
My question is if they were built on BAK4 or BK7-type prisms. Were there some among the countless Japanese manufacturers who typically used BAK4 while others used BK7 prisms?
What type of prisms were used by Carl Zeiss Jena in their 8x30, 7x50 and 10x50 binos?
What about the JB-23 Columbus 7x50 binos? BAK4 or BK7?
12 “‘BACK 4 PRISMS’ ARE MUCH BETTER THAN THE OLD BK7 PRISMS.”
This is one of the harder topics to explain because those who are the most ardent about technical accuracy often lack the experience to determine what is or is not … technically accurate.
Who’s on First?
The first thing on which the inquisitive observer must decide is the real designation of the glass type being discussed. BaK4 (Barium Light Crown by Schott, now N-BaK4, nd1.569) is often referred to by a number of permutations of letters and numerals, including just calling it “back four” as many neophytes do. Also, some designations refer to glass produced by more recently established manufacturers, which may not offer the consistency in quality control one would find associated with long-time industry leaders. It gets even more convoluted when we realize some people talk of glass such as BK6 as if it were Schott Bk6; it’s not. BK6 (note the capital “K”) is made at the Lytkarino Optical Glass Factory near Moscow.
Bk7 vs. BaK4
The big question is whether or not BaK4 is a better glass than Bk7. The short answer is: absolutely not. A highly refined glass, Bk7 has had a long history of being the leading crown glass used in the production of photo-visual instruments.
So just why all the speculations? Having a higher index of refraction (1.569 vs. 1.517 for Bk7), BaK4 enables a total internal reflection of rays at steeper incident angles, and thus avoids a loss of light off axis in short focal ratio systems and most handheld Porro prism binoculars operate around f/4.
The illustrations on the next page describe what any aficionado or shopper can readily see and is something retailers wanting to sell a more expensive binocular are more than willing to show them. Nevertheless, while the images are telling, they might not be telling you as much as you want to know.
A Touch of Realism
During the day our irises are constricted and most or all of the light rays from the grayed out part of the binocular’s exit pupil are falling on the iris and not reaching the retina. At night, when our eyes are dark adapted and our pupils open their widest, our vision is most sensitive peripherally. We do take in light from those gray areas but, as experienced amateur astronomers tell us, a light drop-off of 40% at the edge of our field of view is imperceptible to most people. :cat:
