am805 large bookmark magnifying glass

fresnel lenses are made of pvc materials according to the principle invented by french optical physicists fresnel. the surface of the lens (0.45-0.5mm thick) burns a circle of concentric circles from small to large, outward from shallow to deep, which looks serrated from the profile. there are many circular lines and the dense sensing angle is large, and the focal length is far; the deep sensing distance recorded by the circular line is far, and the focal length is close. several concentric rings in the same row form a vertical sensing area, and a horizontal sensing section is formed between concentric rings. the more the vertical sensing area is, the greater the vertical sensing angle is; the longer the lens is, the more the sensing section is, the greater the horizontal sensing angle is.