The flux for the sun on the sensor/film/whatever is:
Φ/4α² * 1/A²
Where Φ is the sun flux at the surface of the earth, and α is the angular diameter of the sun. A is the f-number of the lens, the only variable in this equation. It doesn't depend on focal length.
The total power over the film is:
Φ/4πα * f²/A²
which does depend on focal length, but that is usually not the determinant factor, as for surface burns the damage depends mostly on the flux.
The flux for the sun on the sensor/film/whatever is:
Where Φ is the sun flux at the surface of the earth, and α is the angular diameter of the sun. A is the f-number of the lens, the only variable in this equation. It doesn't depend on focal length.The total power over the film is:
which does depend on focal length, but that is usually not the determinant factor, as for surface burns the damage depends mostly on the flux.