Leo Lichtman wrote:
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.

Yes, and I allowed for the possibility of that in the part of the post
you clipped (though without the certainty).

However the power of the lens is limited. At some point the number of
incident photons becomes small enough and the ability of the lens to
focus them to a point limited enough, that the power density of the
image on the retina can no longer exceed the threshold of damage. This
is clearly the case when looking at stars other than our sun; at what
point it becomes true for a welding arc I don't know.

It's also going to depend on the size of the pupil, which is going to
depend on what impression of light levels the eye has adjusted to -
being suddenly flashed at night is presumably worse than the same arc
at the same distance on a sunny day.