What focal length should I use to avoid lens distortion?

You may use a super wide-angle camera lens, and the image will be distorted only if you are close to the person being photographed. A feature of photographic lens is that it is "near big and far small", which is not obvious on long focal length lens, but it is often seen on wide-angle lens. The larger the field of view of the lens, the closer it is to the shot scene, and the more obvious the scene deformation. When taking photos of people (especially group photos) or news reports, experienced photographers will try to avoid using super wide-angle lenses with large field of view. According to most people's experience, 135 camera uses a lens with a focal length of about 35mm, digital SLR camera with APS-C size sensor uses a lens with a focal length of about 24mm, and 120 camera (6cm×6cm negative) uses a lens with a focal length of about 60mm, which can avoid the deformation of the subject standing on the side when taking a group photo.