What is lens distortion?

The reason for the distortion is the irregular refraction of the lens caused by the passage of light, but because this is the inherent characteristic of the lens, there is no way to eliminate the distortion in this case, only to improve it. For better lenses, due to the exquisite optical design and materials, the distortion can be controlled in a small case, but it can not be completely eliminated by adopting the optimized design of lens group and selecting high-quality optical glass to make lenses.