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A comment on Rental camera gear destroyed by the 2017 eclipse. Parent comment

Re:Rental camera gear destroyed by the 2017 eclipse by tigershark on Saturday September 02, 2017 @12:55PM (#15156992)
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.
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Re:Rental camera gear destroyed by the 2017 eclipse by 4ad on Saturday September 02, 2017 @02:09PM (#15157382)
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.
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