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 _______________________________________________ Post to the list as PLAIN TEXT only - no HTML Pinhole-Discussion mailing list Pinhole-Discussion@p at ??????? unsubscribe or change your account at http://www.???????/discussion/
