On Tuesday, May 10, 2022 at 11:39:45 PM UTC-6, RichA wrote:
I'm still amazed there was enough light from the sun out there to get those images.
That's a good question.
Light follows the inverse-square law; we know that there's enough sunlight illuminating
Pluto that we can see Pluto from Earth in a good telescope.
Pluto orbits the Sun at an average distance of 39 astronomical units. But it can get closer
to the Sun than Neptune at 30 astronomical units during part of its orbit.
In any case, 39 squared is 1,521. So on Pluto, the light is only 1/1500 what we get
on Earth.
On Earth, on a bright sunny day at the beach, if you have a camera loaded with film at ASA 25, and have your lens set at f/16, an exposure of 1/60 of a second should be more than adequate. At least those were the figures I had from memory, although looking at images of old exposure calculators seems to imply that these settings would result in a gross over-exposure, and I need to stop down
about three stops.
If you change the film to ASA 1000 - and they had film that sensitive, just before
people stopped using the stuff, you have already multiplied your sensitivity
to light by 40. Change the lens setting to f/1.4, and you have multiplied your sensitivity to light by 128. We can change that to 64 by only going to f/2, or to 32 by going to f/2.8.
So the fact that the human eye works on a logarithmic scale - and so our photographic technology had to be adapted to the large range of conditions under which we're able to see - means that, yes, the light is adequate on Pluto to take pictures.
John Savard
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