Why are the lenses of cameras and projectors a group of lenses instead of a single convex lens?

Various lenses are combined to achieve the best imaging effect, and zooming and focusing are realized through the displacement between lenses. So you can't use a convex lens. Due to the use of aspheric lenses, lenses with special optical characteristics can be made. Using aspherical lenses means that the lens can be made smaller and lighter, which is usually better than similar lenses that only use spherical lenses. Nikon introduced the first aspherical lens camera lens with 1968: 10mm F5.6 OP fisheye. The world's fastest 28mm lens (AF Nikkor 28mm f/ 1.4D) adopts aspherical lens to ensure its compact structure. By eliminating radial latent light or arrow coma, it can even achieve excellent performance with its maximum aperture. The surface of aspheric lens is an ideal curved surface, which can correct these aberrations. Aspheric lenses-even with the largest aperture-can actually eliminate coma and other types of lens aberrations. They are very useful for correcting the image distortion formed by wide-angle lenses. Nikon adopted three kinds of aspheric lenses: precision grinding aspheric lenses is the highest expression of lens manufacturing technology and requires extremely strict manufacturing standards. Hybrid lenses are made by casting special plastic on optical lenses. Aspheric lens with molded lens is manufactured by using special metal mold technology to mold a specific type of optical lens. Auto-focus DC- nikkor lens, one of Nikon's unique contributions to portrait photography is Nikon's proprietary defocused image control DC (defocused image control) technology. This innovative Nikon technology allows users of autofocus DC- nikkor lens to control the spherical aberration of the foreground or background by rotating the DC ring of the lens, so as to accurately control the ambiguity of the background and foreground, and thus shoot beautiful human images. This will create a circular blur suitable for portrait shooting. These lenses are unique to Nikon. AF-S nikkor lens Nikon's AF-S technology involves ultra-telephoto lenses, such as 300mm, 400mm, 500mm and 600mm, and fast zoom lenses with maximum aperture, such as 17-35mm, 28-70mm and 80-200 mm, which enables these lenses to perform autofocus operation quickly and quietly, thus being suitable for shooting sports and operating scenes quickly. ?