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.
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.
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.
SO is Bak4 better than Bk7?
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.
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.
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.
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.