> OK I get it. Imagine a gazzilion straight lines passing through the hole
> from the subject to the film. If it's light at one end (subject) it will be
> light at the other end (film), and if it's dark at one end it will be dark at
> the other. Eureka, an image!
I think you've got it!
>
> << Now, has anyone tried building the opposite of a pinhole camera? You
> should be able to get an image from the shadow cast by a pin speck. The
> physics is the same.
> >>
>
> Let me see if I understand. The speck would cast a shadow "at a continuum of
> points across the image plane." Meaning that if it was light at the subject
> end of the continuum, it would be dark at the film end (because of the
> shadow), and if it was dark at the subject end of the continuum, at the film
> end it would be..., umm..., wait a second..., don't tell me...
Remember that to have an image you need other parts of the scene that are
bright. As you point out, if there is no light source opposite the pin
speck no shadow is cast on the image plane. The pin speck at this point is
not blocking the light from the other parts of the subject, so the bright
parts of the subject are illuminating this area. This means the image
plane is illuminated by ALL of the bright regions in the scene when
opposite a totally black point in the subject. Now if you move to a point
in the image plane that puts the pin speck between you and a bright
subject region, that source of light to the image plane is blocked. The
illumination in this shadow region will simply be the amount of light
coming from the rest of the subject that can be seen from this point.
Consequently, the brightest parts of the subject will cast the darkest
shadows, and the darkest parts of the subject will get the most overall
illumination.
- Wayde
(wallen@boulder.nist.gov)