What is the difference between BAK4 and BK7 in JAXY Binoculars?

Update time : 2025-07-31 16:48:16

What is the difference between BAK4 and BK7 in JAXY Binoculars?
Porro prisms often use two types of glass: BAK4 and BK7. The former has a higher refractive index, allowing total internal reflection across the entire field of view, while the latter has a lower refractive index, resulting in partial light loss at the edges of the field of view due to incomplete total internal reflection. Therefore, the imaging effect at the center of the field of view is the same for both materials, but the edges will appear slightly darker with BK7 compared to BAK4.

However, since the human eye primarily focuses on the center of the field of view, the difference in imaging quality between the two materials is not significant. BAK4 prisms are more expensive to manufacture than BK7 prisms, which is why lower-end Porro prism binoculars often use BK7 prisms.

When using a handheld telescope, if you hold it 30 centimeters away from your eye and observe the exit pupil, a bright circle indicates the use of BAK4 prisms, while a circle with four dark corners indicates a roof prism. For roof prisms, the choice of material is less critical. Many roof prisms claim to use BAK4 glass, but this is mostly for marketing purposes and has little technical significance. It is also impossible to determine the material used by visual inspection alone.