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.

Previous : Should you choose binoculars or a monocular?

Next : Do you know What is Coating (binouclars)? What are the Functions of Coating?

Not every binocular factory can pass a Disney audit.

Sep .30.2026

BSCI-audited and ISO 9001-certified，JAXY BINOCULARS for your right choice,ED binoculars,birdwatching binoculars,compact binoculars,toy binoculars for kids...night vision scope,thermal device...

Busy period,rush hour production before and after the holidays

Sep .21.2026

Busy period,rush hour before and after the holidays on our JAXY Binoculars,magnfier,compass...

The Total Solar Eclipse of August 2, 2027

Sep .11.2026

2026’s solar eclipse has come and gone. Next big sky show: the total solar eclipse on August 2, 2027.
The full eclipse path runs from Gibraltar through Morocco, Libya, Egypt, Saudi Arabia to Somalia.
Only people in this narrow band get to see the sun totally blocked by the moon.
Are you planning your eclipse trip?

Know how on JAXY Coin-Operated Binoculars ?

Aug .28.2026

Coin operated binoculars,non coin operated binoculars telescope

Felix JAXY

Currency

Currency

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