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Bak6?

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#1 Guest_**DONOTDELETE**_*

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Posted 14 December 2003 - 05:51 PM

Today I saw a pair of zoom binos advertised in one of the Sunday papers. They seem to be of questionable quality, but it did mention they had Bak6 prisms. I've never heard of Bak6 prisms. How do these compare to Bak4, or was this a typing error? :confused:

#2 KennyJ

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Posted 15 December 2003 - 01:35 PM

Hi Alfie,

My guess is that it will be either a typo or more likely a ridiculous attempt to blind the novice with science.

There are indeed many more typs of glass than the widely used BAK4 and Bk 7 -- most of which is more difficult to utilise ( hence more expensive ) for binocular lenses.

Some glass types do have superior qualities to those we are more familiar with -- but by all ccounts form those who know better than I do , hardly worth the difference in price and practicality.

I might add that some very good binoculars have lenses constructed from BK7 glass nd it is my opinion ( only ) that this whole subject is slightly over -hyped.

Quality , number and extent of coatings is probably much more important in reality.

Regads -- Kenny

#3 Guest_**DONOTDELETE**_*

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Posted 15 December 2003 - 04:26 PM

Ok Kenny, I reckon you're right about the typo. Since the make of these binos was Horizon, they sound as if they've come from the Chinese market and had a brand stamped on them. Not knocking all the Chinese optics (as I've got the Oby 20x90s and love them) but it's possible to get some quite poor stuff from that corner of the world as well.

Cheers Alfie

#4 edcannon

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Posted 16 December 2003 - 08:58 PM

I might add that some very good binoculars have lenses constructed from BK7 glass nd it is my opinion ( only ) that this whole subject is slightly over -hyped.


Kenny, based on a few years of using 10x50s with what appear to be BK-7 prisms (diamond-shaped exit pupils), I tend to agree, because I sure can see plenty of faint stars with them. (I need to get charts with fainter stars or at least try EdZ's magnitude test with M45 on them, I guess.) Since I haven't used otherwise equivalent 10x50s that are BaK-4, I don't know for sure how good or bad they are.

Can anything else cause diamond-shaped exit pupils?

Has there been an actual comparison of magnitude reach of very similar binoculars, one BK-7 and one BaK-4?

Ed Cannon - ecannon@mail.utexas.edu - Austin, Texas, USA


#5 brocknroller

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Posted 20 December 2003 - 12:04 PM

From when you first start reading about bins, you get it drilled in you over and over again that BK-7 prisms should be refused at all costs. Well, for $15 the pair of Sears 7X50s I bought on Astromart was an "offer I couldn't refuse" even though they had BK-7 prisms with the characteristic diamond shaped exit pupils. I bought them for kids to use at my club's public Skywatches since they are the lightest 7X50s I've ever seen (about 22 oz.) -- glass lenses -- and I wouldn't have to worry if some kid dropped them. I wasn't expecting much, and like most old bins I've bought, the Sears 7X50 was out of collimation (more noticeable at long range so I will have to collimate them). However, the optics are surprisingly sharp and bright. Given this, I could kick myself for not buying a pair of 20X80 Swift Satellites last year that were for sale at a gun shop for $200. As soon as I saw the square exit pupils, I decided not to buy, based on the no BK-7s dictum. (I called Swift and was told that they import both a BAK-4 and BK-7 version of the Satellite, however, the prism type is not indicated on the binocular). The moon looked fantastic with the Swifts. Best I've ever seen, better than my Burgess 20X80. So I agree with Kenny. Seeing is believing, regardless if the bin has BAK-4 or BK-7 prisms. Try them out and see how they look for yourself rather than automatically following the "never buy BK-7" dictum. Price is also a consideration, BK-7s are almost always cheaper than BAK-4 bins. Bushnell sells a waterproof 8X42 for $65 with BK-7 prisms (demos sell for $45). I haven't used them so I can't recommend them, but that's a heck of a deal for a waterproof bin. As far as other reasons for diamond shaped exit pupils, if a BAK-4 prism is misaligned and intrudes into the light path, you can sometimes see part of a diamond shape cutting into the exit pupil on one side of the exit pupil. I saw this on the Swift Audubon I had. Oddly enough, when I asked a Swift technician about this, he said it was "normal" for the prisms on the Audubon to show in the exit pupil because they were oversized prisms. However, he was at a loss to explain why I could only see this on one side. I guess the prism that WASN'T intruding into the lightpath was misaligned. :-) Brock


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