Focal length has nothing to do with it, in fact, longer focal lengths are generally safer because lenses are slower. Aperture is what matters, lenses gather much more light than our bare eyes.
Is the issue aperture or aperture over focal length? The actual aperture (squared) determines how much solar energy you are gathering. The aperture over focal length (again squared) determines the energy per surface unit.
I'm not sure which is more damaging here.