Why do the people next to me sometimes deform when taking photos?

This situation belongs to lens distortion. Generally speaking, lens distortion is actually the general term for the inherent perspective distortion of optical lenses, that is, the distortion caused by perspective. This distortion is very important for the imaging quality of photos. Disadvantageous, after all, the purpose of photography is to reproduce, not exaggerate, but because this is an inherent characteristic of the lens (convex lenses gather light, concave lenses diverge light), it cannot be eliminated, but can only be improved. The optical design and materials of high-end lenses are exquisite. By optimizing the design of the lens set and selecting high-quality optical glass (such as fluorite glass) to make lenses, perspective distortion can be reduced to a very low level. However, it is impossible to completely eliminate distortion. The highest quality lenses currently tested under extremely strict conditions will also produce varying degrees of deformation and distortion at the edges of the lens.