Porro prisms often use two types of lenses, namely BAK4 and BK7. The usual BAK4 and BK7 refer to the name of Schott glass. They have corresponding Chinese names BAK7 and K9. China also has BAK4, which has a slightly lower refractive index than China's BAK7. So: BAK4=BAK7; BK7=K9.

The former has a high refractive index and the entire field of view can achieve total reflection, while the latter has a low refractive index and the light at the edge of the field of view cannot be totally reflected. Therefore, the imaging effect in the center of the field of view is the same, and the image at the edge of the field of view will be darker with BK7 than with BAK4. However, the attention of the human eye is mainly concentrated in the center of the field of view, so there is little difference in the imaging effects of the prisms of the two materials. The ex-factory price of BAK4 prisms is more expensive than that of BK7, so low-end Porro prism binoculars often use BK7 prisms. Hold the telescope, keep your eyes 30 cm away from the eyepiece, and observe the shape of the exit pupil spot. If it is a bright circle, it is a BAK4 prism. If it is a circle with 4 dark corners, it is a roof prism. For roof prisms, it doesn’t matter what material is used. Many roof prisms also claim to use BAK4 prisms, which are basically commercial propaganda without technical significance, and it is impossible to judge what material is used with the naked eye.