[4ad]

Correct.

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.