On Fri, 5 Dec 2003 17:54:36 -0500, "Walt Wayman" <hiwayman@xxxxxxxxx>
wrote:
>I admit to being simple-minded. So, I think of it like this:
>
>A tiny little man lives in my camera.
>
>He knows what kind of film I have loaded. He understands ASA/ISO
>numbers.
>
>His job is to control the blinds that cover the window where the
>film goes by.
>
>If the lever on top of the camera is set to the "auto" position,
>when the shutter button is pushed down and the mirror thingy flips
>up,
After this, on AUTO, I think it's just a little different!
The first curtain opens and, as it starts to move, the little man
measures the light being reflected from the pattern on the first
blind. He integrates it with respect to time to get the total quantity
of light that falls on the blind and film. He knows what ASA setting
has been dialled in and when he decides that a sufficient quantity of
light has come through, he tells the second curtain to start closing.
His job is now done and he resets all his measurements and waits for
the next exposure. If the light is very dim, it takes much longer for
the total amount of light to come through so he doesn't let the second
curtain start to close just yet. He keeps measuring, now looking at
the light reflected off the film surface. When he's happy that enough
light has reached the film, he tells the second curtain to close.
(This might be tens of seconds).
If he finds a dedicated flash on the camera he does his job a bit
differently. He lets the first curtain open fully and fires the
flash; he then starts measuring and as soon as enough light has come
through he sends the flash-quench signal to stop the flash. This only
takes less than a millisecond, so he tells the second curtain to close
as soon as the first is fully open.
He doesn't need to know anything about the lens or its aperture to do
this. It can be a pinhole or anything else. All he cares about is that
sufficient light must reach the film before he terminates the
exposure. He's so smart that if the light level is low (so that he
might not let the second curtain close for several seconds) but
someone happens to fire a flashgun at the scene (another photographer
even) he's include this in his measurement and close the second
curtain sooner.
Walt's style of explanation is great and reminds me of a book I had
some decades ago (!) as an electronics student. It was written by an
American professor, H H Skilling, and contained a lovely explanation
of an induction motor where a little intelligent bug sat on the rotor,
watching and measuring the magnetic flux wave going past him.
John Gruffydd (Mold, Wales, UK)
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