[patcheudor]

Yeah, if you use a super-telephoto lens, then take your camera body off and change the focal point so that you are using your lens as a magnifying glass like they did in the video. No doubt. For the rest of the sane world; however, it's not quite that risky. For reference, that Leica I used in my shot is a one of a kind as it was the first lens to roll off the line when Leitz branding was changed to Leica & estimated to be worth between $12,000 to $22,000. I wouldn't have risked it if there was any risk whatsoever.

[tigershark]

No, it is the opposite. A 50mm f/1.4 lens focus the light and the heath much more than a 300mm f/5.6. You were quite lucky with your camera I guess.

[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.

[4ad]

Surface damage mostly depends on the power density, not on total power, so it depends mostly on the aperture, not on the focal length. This makes longer focal lengths safer to point at the sun because they are slower lenses. They are also safer if you don't explicitly point them at the sun because they are less likely to catch the sun in the smaller field of view.

Burned shutter cloths on Leica M series cameras with wide angle lenses is a real problem that can happen simply if you leave the camera without a lens cap lying on a table. This is less of a problem on most other cameras, as they use metal shutter curtains.