It's the bending of light.
Just as a prism bends light, and then breaks it into different colors in
a rainbow, the edge of a pinhole bends light and breaks it up. The
rainbow from a prism is focused, white light that becomes a blurry
colored light band because different wavelengths of light get bent
differently, and travel at different speeds through a solid. (For the
physicists out there, don't bother to email me back to correct this, I
know I over-simplified it...in a way that will make it easy to use the
idea. This is about photography, not optics.) Different colors of
light focus in different ways - the perfect focus setting for pure red
light is different than for pure blue, than for pure green. Lenses are
basically a form of prism, and bend light (light gets bent at each
materials interface it goes through) to get it to focus an image to a
certain size at a certain location, and in camera lenses, some of the
glass elements are bending the light to size the image and focus it in a
certain way, other glass elements are there specifically to get the red,
green, and blue light colors back in line with each other to reduce the
impact on sharpness - manage the negative impacts of diffraction. Glass
lenses bend the light, and then rebend it to try to get it lined up
correctly.
If it's not done perfectly, you'll see little 1960's colored psychedelic
auras around the edges of things. It's called chromatic aberration. In
black and white, it just looks unsharp.
In a lens, despite all the hard work with the glass elements to get the
light all lined up, stopping the lens down to a small aperture basically
puts a pinhole (which diffracts light) in to the middle of a system of
lenses designed to diffract light at some optimal aperture. All lenses
have apertures where the lens produces the sharpest result. Those are
the apertures where the designers optimized the glass design to work
with the aperture size. Bigger apertures will have a little more blur
because the glass won't focus all the colors as well; smaller ones will
have a little more blur because the small aperture bends the colors of
light more unevenly than the designer anticipated. Once you learn the
characteristics of your lens, you try to shoot at the best apertures.
A pinhole camera causes diffraction blur along the edges, but you can
minimize its impact on your image a lot by 1) using a really big
negative so that the ratio of light blur to size of image is small and
seemingly not noticeable; 2) shooting in black and white. And once you
learn the diffraction and chromatic aberration characteristics of your
pinhole, you'll learn to use them artistically.
So, the blurry fringe that you see along a well-defined edge, is the
result of diffraction bending light, and then causing what looks like
unsharpness along what you expect to be a sharp edge because it bent the
colors unevenly and they hit the film at different focus points. The
unsharp mask in Photoshop, or in the chemical photography world, is a
way to make that blurriness go away a bit.
CCDs in digital cameras use little tiny lenses over every color sensor,
to intensify the light, and they are therefore often subject to
diffraction color auras, or chromatic aberration, no matter how
wonderful and expensive the lens on the camera itself.
Ed Nazarko
-Original Message-
From: pinhole-discussion-admin@p at ???
[mailto:pinhole-discussion-admin@p at ???] On Behalf Of Lisa Reddig
Sent: Wednesday, December 11, 2002 11:39 AM
To: pinhole-discussion@p at ???
Subject: [pinhole-discussion] What is Diffraction?
Ed Nazarko writes:
Diffraction cannot be avoided, it's the way light behaves when going
through any
system. Every lens, telescope mirror and pinhole has diffraction.
The
best optics
are said to be 'diffraction limited' which means that the optics are
about as good
as they can be because the other defects in the system have been
reduced
to below
the level of the diffraction.
I've been hunting on the web for a good description of diffraction, but
it
all talks about physics and x-rays. Could some one give a real simple,
basic description of diffraction and how it shows itself in pinholes? I
would appreciate no equations if possible. An example with a picture
would
be cool.
Thanks
Lisa
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