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  Brunton literature describes some of their new binoculars as having SF prisms which are boasted to be superior to those constructed of BAK 4 and of course Bk7 glass .

  I recently came across a list of the various consitutents and relative qualities of optic glass and was quite surprised to learn that in a "league table" of around 20 different kinds in general use , BAK 4 was well below half way down the list , indicating to a layman such as myself that there are at least 12 types of glass that would perform better used in  optic prisms. Indeed SF2 as it is listed therein appears higher up the quality chart , although incidentally I noticed that quite a few of these "high performers" contain arsenic and other "nasty" ingredients .

I have 3 questions really .

1. I'm guessing that factors such as relative cost , availability , fragility and health and safety issues restrict the use of many of these compounds and thus influence designer and producer decisions .Would this be an accurate assumption / conclusion to draw or am I suffering from "burger slinging "

mentality , as WJC so amusingly puts it ?

2. In the case of Brunton , which I believe to be currently producing ( or at least a name on ) some very fine binoculars , is this likely to be a step forwards and away from other leading manufacturers , or just a gimmick ?

3. Why is it that no other manufacturer seems to have tried this ?



Brunton X105 Epoch 10.5x43 Binoculars   $1400.

SF prism material is better than any glass on any other binocular;  58-74mm interpupillary range;  Hybrid aspheric Ocular lenses


Brunton X75 Epoch 7.5 x 43 Binocular   $1200.

The crown jewel of the Brunton Epoch is the SF roof prism—a prism previously only used in high-powered telescopes. Most binoculars use the excellent Bak-4 prism, but the SF is simply a step above.....the variable speed focus is finer and slower from 30 feet in, and lightning fast from 30 feet out.....near focus is 36 inches.

These look like nice binoculars, listed on many retail pages.  

There is a review on the National Rifle Association site: http://www.nrahq.org/publications/tar/dopebag/db19.asp

I'm not sure if the 'SF' prism is good for performance, or for marketing.  I'd like to know.

There are glasses that provide desirable optical qualities in a binocular prism, for example I believe a higher index of refraction allows use of a smaller prism while providing full field illumination.  But a binocular has to be built of glass that is resistant to moisture & air, and takes a good polish; and fits in a budget; among other qualities; and I believe BAK 4 is basically unsurpassed.    --Peter



Binocular List #267: 02 September 2003


I really let the email pile up in the past 1.5 weeks.  There are a lot of good questions in this list, please let us know some answers & I'll put out the next list much sooner.   --Peter


Subject: Translation

From: <mikedenmark@___ele.dk>

This is the last of 3 reports, I have found in the danish archives, related to Zeiss and Norinab.

In the future, I may return with translation on reports, made by workers, who has been sent on training at the Goerz and Zeiss Factories around 1906.

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