What are Bak4 and bak7 lenses?

Question

...when buying a binocular

Answer 1

Here are some links for buying binoculars. They include explanations of the lens types you are asking about.

http://www.chuckhawks.com/binocular_basi...

And from:

http://www.optcorp.com/info_choosebinocu...

"WHAT'S THE BIG DEAL WITH BAK-4 PRISMS?

Optical glass quality varies widely between models, this is one of the many reasons for the wide range in price that you will find in the stores ($30 - $2000+). The less expensive binoculars generally are manufactured with BK-7 prisms. If you turn your binoculars around and look down the front lens towards the inside, you can see the difference between BK-7 and BAK-4 (much better prism quality). If the binoculars have BK-7 prisms, you can see a squared off side to the general roundness of the image. BAK-4 prisms show a truer round, which translates to better light transmission and edge-to-edge sharpness."

Hope that helps.

Answer 2

BAK4 and BK7 are not about the lenses, they are about the prisms that fold the light path inside the binoculars. BAK4 prisms are generally considered better because they give a perfectly round 'exit pupil' (what you actually see) and the BK7 prisms give a slightly square exit pupil.

The differences are in what type of glass the prisms are made from: BAK4 is made from barium crown glass and BK7 from borosilicate glass. They have different indices of refraction, which ultimately gives the differences in image.

Answer 3

different kinds of glass used in the prisms.

bak4 (barium crown) is more expensive but works better. bak7 (borosilicate) is cheaper and doesn't work as well.