"When a beam of light parallel to the main optical axis passes through a convex lens and intersects at a point, this point is called the focal point. The plane passing through the focal point and perpendicular to the optical axis is called the focal plane.". There are two focal points. The focal point in the object space is called the "object focal point", and the focal plane there is called the "object focal plane"; On the contrary, the focal point in the image space is called the "image focal point", and the focal plane there is called the "image focal plane.".

Optical glass is the main raw material for lens production due to its high transparency, purity, colorless, uniform texture, and good refractive ability. Due to different chemical composition and refractive index, optical glasses include flint glass, crown glass, and lanthanum crown glass.