[Tfug] Way OT: optics mumbo jumbo

[jblais]

> > manageable.
> > > >
> > > > Inverse 4th power (x^(-4)), because the light
> > makes
> > > > a round trip
> > > > from a light-dispersing object.
> >
> > Well -- I think the exposure setting for the camera
> > taking a picture of an
> > object, only depends on the distance of the light
> > source to the subject.
> > The camera can be anywhere, so it's really just the
> > inverse-squared law.
>
> But doesn't the light then have to make it's
> way *from* the object *to* the camera?  I.e.,
> (thought experiment), if I was sitting on the
> MOON trying to take a photo of that object
> sitting (on Earth) in my family room, wouldn't
> there be far less light reaching my lens??
> *Regardless* of how close the light was to the
> subject??
>
> --don
>

Think of a lit candle, or of a subject that's illuminated to be as bright as
a candle.
It scatters off light in every which way.  If you hold a card that is 1-ft
by 1-ft, at a distance of 1-ft, count all the little photons hitting it in
one second.  Call that one lumen worth of the little guys.  Now that is 1
lumen per 144 sq inches.  Now take a camera with a lens aperature of 1 sq
inch and it will collect 1/144 lumen.  And say it focuses the candle to
1/10in x 1/10in, or 1/100 sq in. In one second exposure you then get 1/144
lumen per 1/100 sq inch or 100/144 lumen per sq inch.  Notice how the lens
magnifies, kind of like burning paper with a lens pointing to the sun.

So now move the camera back another foot.  The light previously falling on
1ftx1ft, now falls on 2ftx2ft so that 1 lumen per sq ft is now 1 lumen per 4
sq ft, or 1/(4*144) lumen per square inch (inverse square law).  Now get the
camera again with the 1 sq inch aperature, and set it at the 2ft distance as
well.  You now get 1/(4*144) lumnes through the aperature, BUT the image is
now smaller!  The image, because the distance was doubled, is now 1/2 the
size, so it's 1/20in x 1/20in (again inverse square law).  So now you have
those 1/(4*144) lumens per 1/(20x20) image, or 400/(4*144), or 100/144
lumens per square inch -- the same brightness of the image, so use the same
exposure.

Take a picture of something here on earth.  Use f16 at the ISO rating of the
film (funny how that works, perhaps f22 for Arizona).  You'll get a good
exposure.  Now use the same exposure settings foa a pictuer of the moon, at
night.  f16 and the speed set to the ISO of the film.  My guess is that
you'll get a fairly dark smudge.  Now go to the NASA web site and find a
picture of a moon rock.  It will probably look like a similar dark smudge.

Joe