What is the principle of the telescope head of iphone?

Optical zoom: Optical zoom changes the focal length of the lens by changing the relative position of each lens in the lens. Digital zoom changes the size of the imaging surface. In fact, digital zoom does not change the focal length of the lens, but changes the viewing angle by changing the diagonal of the imaging plane. Optical zoom will not damage the image quality, while digital zoom will reduce the image quality.

However, the market is mixed, and some manufacturers counterfeit or seek the maximum cost without considering the imaging quality. Once the lens is used, it will reduce the clarity of the image?