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Bak-4, Bak-7, Roof Prisims

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#1 GeneT

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Posted 15 February 2010 - 04:57 PM

I recently received my Eagle Optics Ranger SRT 10x50 Binoculars with Bak-4 and roof prisms. Could/would someone please explain the difference between Bak-4 and Bak-7 prisms, and roof prisms? This is my first foray into using binoculars for astronomy, to augment my two telescopes. By the way, these binoculars are excellent. Objects are bright,clear, and in focus across the entire field. One of my 'drivers' was any binoculars I purchased had to be usable with glasses. The usable eye relief easily accommodated my wearing glasses. To any one who answeres the above, thanks!

#2 Erik D

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Posted 15 February 2010 - 05:16 PM

"Best of" :


Bak 4/BK 7, Roofs/Porros

ERik D

#3 KennyJ

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Posted 15 February 2010 - 05:21 PM

Bak4 and Bk7 -- different types of glass --

I could spend an hour typing but it's easier just to suggest you either type the words into google search engine or guide you to the link below , to understand the differences and why both are very good types of glass for different situations .

http://www.birdforum...ad.php?t=156753

Roof prism binoculars are a TYPE of binoculars , different especially to Porro prisms .

A VERY basic explanation is provided here :

http://support.nikon...nd-roof-prisms?

Kenny

#4 BillC

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Posted 15 February 2010 - 06:07 PM

There is no "Bak7" glass in the Schott catalog!

Cheers,

Bill

#5 hallelujah

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Posted 15 February 2010 - 06:17 PM

Did you mean catalog? :o

#6 pcad

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Posted 15 February 2010 - 06:40 PM

When choosing which glass to use for binocular prisms the idea is to take advantage of total internal reflection as much as possible. This boils down to matching the index of refraction to the focal ratio of the objectives. BaK-4 is appropriate with faster optics, BK-7 with slower optics. BaK-4 can be used with slower optics also, but BK-7 can't maintain total internal reflectance with faster optics. When BK-7 is used with fast optics you may find parts of the exit pupil less illuminated than others. It doesn't make the image blurry, but perhaps dimmer than it would be otherwise.

Roof prisms are prisms where 2 faces meet at a mutual edge. The geometry of these faces and edge have to be made to high standards since the light being focused will cover both faces and edge at the same time. In the two most common roof prism designs used for binoculars there's only a single pair of roof surfaces. There are other reflections which have to occur as well. Both of these designs use 2 prisms for each side of the binoculars.

When roof prisms are used the way they are in binoculars there is destructive interference due to half the light having a different phase than the other half. This can be decreased by using phase correction coatings on the roof faces(face?). The roof edge may also cause some spiking on bright objects. This is reduced by making the edge as fine as possible.

Porro prism clusters differ in that prism edges never bisect the light path. They also use 2 prisms per side. They don't have phase interference or roof edges to worry about.

#7 BillC

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Posted 15 February 2010 - 06:53 PM

Post deleted by BillC

#8 KennyJ

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Posted 15 February 2010 - 07:02 PM

Bill - < There is no "Bak7" glass in the Schott catelog! >

Stan - < Did you mean catalog? >

Were either of you refering to catalogue ? :-)

Kenny

#9 pcad

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Posted 15 February 2010 - 07:07 PM

Bill, if you think gtown1 is going to correct the title and his post to change Bak-7 to BK-7 perhaps we should just ask him to.

gtown1,

While we understand your question, the two glasses most commonly used for prisms are BaK-4 and BK-7. Perhaps you could edit your post and title to change BaK-7 to BK-7. Thanks.

You're not the first, nor the last to make this typo.

#10 BillC

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Posted 16 February 2010 - 02:20 PM

Typos have become a specialty with me. Also, I have been, privately, taken to task for again pointing out that there is no BaK7 glass.

But, because I say it so much, why is it that some feel the need to brow-beat me with scriptures?

Yes, I admit to being human. And perhaps I grow old bringing up the point many times a year. But, as long as someone learns something, I don't care what is thought of me, personally.

Perhaps if I hadn't used a "!" in my post, I wouldn't have gotten reamed. However, I APOLOGIZE!!! Heck, in order to keep peace, I will gladly apologize for things I didn't even do.

Cheers,

Bill

#11 KennyJ

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Posted 16 February 2010 - 02:30 PM

Speaking of typos , no - one until now has mentioned the word Prisims in the title :-) ( which , incidentally , Peter , I don't think can be edited by an original poster , unless that person is either a moderator or administrator )

Kenny

#12 BillC

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Posted 16 February 2010 - 04:42 PM

You think Bk7 is inferior? Johnny Cash had Folsom Prism Blues . . . Must be a firm I haven't heard of.

Bill

#13 BillC

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Posted 16 February 2010 - 04:52 PM

. . . and I just looked back over the posts and realized that I has probably taken to task (this time) because I posted TWICE! Folks that was NOT my curmudgeonisnosity. In that short a time, I simple forgot that I had ALREADY posted! I hope none of you have a stroke. If you live, you may just have enough going for you to get yourself in trouble!

Cheers, :jump:

Bill

#14 Erik D

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Posted 16 February 2010 - 05:15 PM

However, I APOLOGIZE!!! Heck, in order to keep peace, I will gladly apologize for things I didn't even do.

Cheers,

Bill


Yes Bill, I KNOW in my heart that's right attitude in spousal communications too. But much more difficult to put into practice in the heat of the moment. I am getting better with age though....

:)

ERik D

#15 BillC

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Posted 16 February 2010 - 07:05 PM

I have just come to accept I am a RESPONSIBLE person. If there is a screw-up . . . I'm RESPONSIBLE. See, that wasn't hard; my kids taught me. And taught me. And taught me.

Cheers,

Bill

#16 NDfarmer

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Posted 16 February 2010 - 09:53 PM

I have just come to accept I am a RESPONSIBLE person. If there is a screw-up . . . I'm RESPONSIBLE. See, that wasn't hard; my kids taught me. And taught me. And taught me.

Cheers,

Bill


Bill:

Then what would you say about the differences between Bak-4 and Bak-7. I would really like to know, also.

Jerry

#17 EdZ

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Posted 17 February 2010 - 06:38 AM

what surprises me is that just inches away from this discussion is a library of links to discussions, one of which links to several previous discussions regarding these prism types, and rather than go to those resources and read about it, it will, as is often the case, get rehashed here, again, for the umpteenth time. Oh well, such is life.

edz

#18 RichD

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Posted 17 February 2010 - 06:48 AM

What was Mr Bill saying a few days ago about the ephemeral nature of the net?

;)

#19 Jim Rosenstock

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Posted 17 February 2010 - 11:06 AM

what surprises me is that just inches away from this discussion is a library of links to discussions, one of which links to several previous discussions regarding these prism types, and rather than go to those resources and read about it, it will, as is often the case, get rehashed here, again, for the umpteenth time. Oh well, such is life.

edz


Some people prefer asking questions to doing research. Fortunately, plenty of folks here seem to like answering questions, so it's all good, IMO.

To be fair, some topics have so many threads, posts, and opinions that it's a bit daunting to sort them out. I think sometimes folks post questions that have already been discussed to death, hoping to get a single, concise summary from a knowledgeable person. Often, this works.....but just as often it results in yet another lengthy thread.... :tonofbricks:

As for spelling and typos....I have a bit of "English teacher" in me, and sometimes wince at misspellings of common words. But generally, no harm done. OTOH, mis-typing a technical specification can be problematic. If you take LSD-25 you'll go on a trip. If you take LD25, there's a one in four chance you'll wake up dead! :o

:grin: :grin: :grin:

Jim

#20 94bamf

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Posted 17 February 2010 - 01:28 PM

I have just come to accept I am a RESPONSIBLE person. If there is a screw-up . . . I'm RESPONSIBLE. See, that wasn't hard; my kids taught me. And taught me. And taught me.

Cheers,

Bill


Bill:

Then what would you say about the differences between Bak-4 and Bak-7. I would really like to know, also.

Jerry


Am I the only one who found this post humorous after the discussion above?

Jerry, you can never be too sure of what Bill will say, but I would guess the main difference he would point out between Bak-4 and Bak-7 is that one exists, and the other doesn't... :grin:(atleast that is my understanding of this discussion from previous threads/posts)

I believe the correct term is "Bk7", not "Bak-7"..

Ken

#21 BillC

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Posted 17 February 2010 - 03:05 PM

Watching what I say so no one will be offended--ESPECIALLY when No offense is meant:

The acrhonyms are not to be used interchangebly. As Rush Limbaugh says, "words MEAN things."

Bk, as in Bk7, means borosilicate CROWN glass.

BaK, as in BaK4, means BARIUM LIGHT CROWN glass.

Okay, stand by, I'm gonna get fresh:

Despite the bull we see on this and other lists, one IS NOT BETTER than the other--just different.

What many need to learn is that, at any point in time, hundreds of glass recipies are being produced. They are picked for their particular design properties.

Cheers,

Bill

#22 GeneT

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Posted 17 February 2010 - 04:58 PM

There was a lot of chatter about Bak-4 and Bak-7 instead of Bk-7, and old threads and so on. I simply asked for (if there is one) a brief discussion of the differences. I have been a telescope observer for more than 50 years. Only recently did I become interested in a set of binoculars to augment my astronomy viewing. I purchased a pair of Eagle Optics Rangers 10X50 and noticed they were Bak-4 with roof prisms. I am not familiar with binocular nomenclature. That was the reason for the post.

#23 EdZ

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Posted 17 February 2010 - 06:11 PM

I simply asked for (if there is one) a brief discussion of the differences.



there is. It was captured in the Best Of threads a long time ago. A link was provided. old thread or not, the explanation hasn't changed.

edz

#24 DJB

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Posted 18 February 2010 - 02:52 AM

Hi guys,

Bill C. has been absolutely correct all along.

Now, if you work in a lab environment, like I have, when you must look up a particular glass or coating (in an optical catalog, for example), it must designate EXACTLY what one is looking for. I have to defend Bill 110%.

For the forum, probably we can get around this. However, we all should to strive to explain what it is exactly in the question, or what we are looking for. Easier on all of us. Thanks.

Best regards,
Dave.

#25 Simon S

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Posted 18 February 2010 - 10:18 AM

I find to much emphasis is made to what prism materials are used. It's the same as the first question asked of me when I have my camera with me! How many mega pixels is that? More mega pixels is not good most of the time and not as important as the noise and camera resolution.
I would guess that most reputable manufactures fit what is best for the binocular.






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