Get to know - #4 Bak4 & Bk7 Prisms

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Get to know - 4 Bak4 Bk7
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• Mostly, we get the experience from others, Bak4
  is better than Bk7.
• Is this real? Let's continue to get to know
  prisms.
• This time we will talk about Bak4 Bk7.
• Well, To not be so boring, we first solve the
  most important question.

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HOW TO DISTINGUISH BAK4 BK7?

• In the Porro prism, you can hold the binocular at
  about 40cm to observe through the eyepieces.
• If the eyepiece among the spot shape is a perfect
  circle, congratulations you got a bak4 binocular.
• If it has a gray edge, the binocular is a BK7
  prism.
• This only works to Porro binoculars.

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• Want to know more? Continue for the rest.
• First of all, you should know that Porro and Roof
  are types, while Bak4 Bk7 are models.
• Even Roof has types too. But to make the easy to
  understand. We just talk Porro Roof.
• Just like computers, AMD and Intel are both great
  CPUs, but different brands(types). Meanwhile,
  they are many models of each of them. So are Bak4
  Bk7.

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SO IS BAK4 BETTER THAN BK7?

• It depends.
• Generally, Bak4 light transmission can reach 99,
  Bk7 light transmission rate is about 83. That is
  why you may see some binoculars show their light
  transmission rate of 99.99 with Bak4 prisms.
• But a few binoculars that use the top Bk7 prisms
  are very expensive. They have magnification and
  size beyond the average binoculars, and produced
  with very clear purposes with really good
  quality. Trust me they're more expensive.
• In usage, people cannot distinguish between Bak4
  and Bk7 by the naked eyes, especially roof
  binoculars. Even Porro binoculars with low
  magnification(lower than 5x) or better design,
  because the field of view is enough large and
  can't observe the gray edge phenomenon.
• But the interesting thing is that most people
  can't accept the Bk7 binoculars once they have
  used the bak4 binoculars. It is maybe caused by
  psychological factors. So the purchase advice,
  more recommended BAK4 binoculars.

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Why is bak4 batter?

• Here is a picture from Wikipedia that shows how
  light passes through a prism. Of course, it is
  just a conventional prism for educational
  purposes. Here we can also observe the loss of
  light.

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• And this is what a Porro prism looks like, they
  are always a pair, light pass through two prisms
  like a snake, allowing the human eye to see the
  corrected image.
• As we mentioned before Porro prisms theoretically
  have a 100 refractive index, YES, if the
  incident light is parallel light like the wiki
  picture.

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• In reality, the light passing through the
  objective lens is pooled(Objective lens are
  convex lens), causing a slight difference in the
  angle of incidence of the light. In the
  illustration below, only the yellow light
  completely passes through the prism, while the
  red and blue light is partially lost. How to
  reduce this part of the loss? The answer is
  already in this blog. Go find it.
• Source Uscamel Optics Blog
• https//uscameloptics.com/blogs/optics-101/get-to-
  know-4-bak4-bk7

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Thank you for reading!