Re: Tthe aperture hexagon

jfl...@xxxxxxxxx wrote:
A question that has been in my mind for years - can anyone explain the
hexagonal ghost image that so often shows up in images? I undetstand
it is the shape of the aperture, but why does the image take its
shape?

I remember once, from when there was a partial eclipse in my area,
people were advised that in the shadows cast by trees, the spots
between leaves would take on the shape of the crescent sun.

This would be because where there is a small space between, say, three
leaves, it might act like the pinhole in a pinhole camera.

Yet, the shadow of a tree usually looks like a tree.

That's because the Sun is small enough so that it also can act like a
"pinhole" for an image of the tree, but because the Sun is not a
point, parts of shadows of distant objects will be blurry. Since that
blurring comes from the Sun not being a point, it takes the shape of
the Sun.

Ghost images in a camera result from spurious reflections; so this
involves light that hasn't taken the usual path through the lens. That
is light that hasn't been brought to a focus properly by the lens.

Ideally, a lens takes rays of light coming from each point of whatever
you want to take a picture of, and bends them all so they come back to
a point on your film. The iris diaphragm of the lens is ideally put
where it is in the exact middle of this process, where the rays from
each point in the object that will be used to make the image all fill
the entire diameter of the lens at that spot.

So, when the iris is constricted, it uniformly reduces the amount of
light that gets to the film from each part of the image, whether in
the center or at the edges. It is put at the spot *in* the lens,
therefore, that should be as far out of focus as possible from the
film's viewpoint.

When something goes wrong - when light is reflected from surfaces
within the lens - then the iris diaphragm, in the image created by
that reflected light, is no longer totally out of focus, but other
things - like the Sun - have become out of focus, and so they can fill
an image of the iris with light (instead of one seeing on the film an
image of the Sun).

John Savard

.

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