Mostly, we get the experience from others, Uscamel Optics Bak4 is better than Bk7. Is this real? Let's continue to get to know prisms.This time we will talk about Bak4 & Bk7.

1. Get to know -
#4 Bak4 & Bk7

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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

9. Thank you for
reading!