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Leo Lichtman
 
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Default Safe distance to watch arc welding


wrote: A lens can't collect and focus more energy
than is intercepted by it's total area. (clip)
^^^^^^^^^^^^^^^^^^
That is true. However, a given light source at a greater distance produces
a proportionately smaller image. so the brightness of the image remains the
same. I'm not making this up. Lay out a diagram of a lens and do the ray
tracing, and you'll see.

Or, as experimental evidence, think about what i said about taking a
photograph. Let's say you take a picture of two billboards. One is 100
feet from you, and the other is 1000 feet. They are in the same picture,
and they are both correctly exposed. The more distant billboard has the
same illumination falling on it, but, being 10x farther away, the lens will
receive 1/100 as much light. Inside the camera, the image of the more
distant billboard will be 1/10 as large, so it will have 1/100 as much area.
So the brightness of the image will come out the SAME.