At 3:05 PM +0000 9/22/03, olympus-digest wrote:
>Date: Mon, 22 Sep 2003 11:55:55 +0100 (BST)
>From: julian_davies@xxxxxxxxxxxxxx
>Subject: Re: [OM] Nope, It missed it by few hundred miles
>
>
> > from: Joe Gwinn <joegwinn@xxxxxxxxxxx>
> > date: Mon, 22 Sep 2003 01:55:40
> > to: olympus@xxxxxxxxxxxxxxx
> > subject: Re: [OM] Nope, It missed it by few hundred miles
>
> > For one thing, it takes from 4 to 40 photons absorbed in a
> >certain period of time to render a grain developable, and the
> >developed grain has about 10^6 atoms in it, whereas the above
> >assumes that a single photon can expose a grain, which isn't
> >true and cannot be true. Why cannot it be true? Because
> >random thermal motion would then expose the film, leading to a
> >hopelessly high fog level. Requiring multiple photons arriving
> >within a short period of time sharply reduces the probability
> >that thermal motion will randomly expose grains.
>
>In my defence, I would point out that the photons involved are focused by a
>lens onto an image receptor (film). By definition, therefore, if this system
>is "sharp" and "focused", then all repeats of the two photons over time MUST
>pass through the same points of the film. Therefore we just repeat the example
>40 times.
The ideal lens of geometric optics does that, but no real lens follows this
ideal, for at least two major reasons. First, all practical lenses suffer from
abberations, so even in the geometric theory of image formation, the light rays
don't quite pass through the same mathematical point in space. Close, a matter
of 5 or 10 microns, but not exact. Second, the wave nature of light prevents
focal spots from being much smaller than a wavelength of light in diameter,
about 0.5 micron. Not that any photographic lens is diffraction-limited, so
the abberations dominate.
> >Nor are film grains ever "gray". A grain either develops or not.
>
>So digital photography is not new! You're quite right here, but of course
>clumps give the grey - scale.
Yes, grains are binary (either there or not there), and clumps (random clouds
of grains) are dark gray, but what we see as the density of image areas is the
number of clumps per square millimeter.
>This effect may or may not be true. I read it. It made sense to me. I passed
>it on. Probably badly and using some of the wrong terminology.
>There is an alternative view which may or may not help, and which also may or
>may not be "true":
>In the case where light of varying spatial frequency strikes a film, we know
>that the reflection from the film varies with temporal frequency. What would
>make it vary with spatial frequency? Nothing I can think of.
The spatial frequency is measured across the image, in the plane of the image.
To the eye, it just looks like alternating bars of light and dark. There is no
temporal variation involved in the issue.
>Assuming this is true the light absorbed by the film is constant with spatial
>frequency.
This is true.
>Light within the resolving limit of the film is absorbed by image - forming
>grains.
>Light outside the resolving limit of the film is absorbed by?????
>And no, it doesn't all just sail straight through to the anti - halation
>layer. Statistically SOME of it MUST strike grains.
Yes, it all does. Or, more precisely, the same fraction hits the grains as
always. If the spatial frequency is too high (the bars of light and dark are
too narrow and too closely spaced), the spatial modulation (those bars) simply
averages out to some intermediate shade of gray. Nothing is lost, nothing is
gained. It's just smeared out. The human eye does exactly the same thing, for
exactly the same reason. If we look at a fine pattern on some cloth, from a
distance we see the average color, but up close we can see the pattern.
Joe Gwinn
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