In fact, cameras, telescopes, slide projectors and microscopes are all synthetic lenses composed of convex lenses and concave lenses, which are equivalent to convex lenses with positive focal lengths.
This is because the imaging quality of a lens with only one convex lens is poor, so combining multiple lenses into one lens can improve the aberration and chromatic aberration of a single lens.
The lens focal length of these optical instruments is designed according to different requirements, and it is impossible to generalize.