Correct. It is based on fractions of a wavelength of sodium light (sodium about 500nm I think). It's actually possible to detect the difference between a 1/4 wave optical system and one that is say 1/8 wave by looking at the diffraction ring pattern from a point light source inside and outside of focus. Yet in most cases it's hard to detect the difference in actual use. 1/4th wave is considered "diffraction limited optics" which only means that no errors can be visually detected that are distinguishable from those caused by diffraction effects. Still a lot of people say they can tell the difference in areas of contrast and resolution. Perfect eyesight must help though because even with 20/15 corrected vision I can't. When it comes to the term "APO" in optics, unless you are talking about perfection, optics can be designed to be visual or photographic. Meaning that of the 3 color correction it can be either corrected towards the red end for the human eye or the blue end for film. These days it is possible to correct for just about the entire spectrum if it is important and cost is no object. Kent Gittings Kent Gittings
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Rodger Whitlock Sent: Tuesday, November 27, 2001 11:55 AM To: [EMAIL PROTECTED] Subject: Re: APO Lenses On Mon, 26 Nov 2001 at 19:12:30 -0500, "Isaac Crawford" <[EMAIL PROTECTED]> wrote: > You mean down to the photon!? Who's BSing now? We aren't > talking about a machine shop, we're talking about light. It is > literally impossible to grind lenses to within a photon's breadth > of correction. The criterion isn't a photon's breadth (I'm not even sure a photon has breadth), but rather the wavelength of the light (= photon). As it happens, a mirror ground to within 1/4 lambda (wavelength) behaves as though it were optically exact. I imagine that a similar phenomenon happens with lenses. And, yes, it is possible to form optical surfaces to this accuracy. Kodak has a proprietary method using a beam of argon ions to erode glass surfaces. This method has been used to correct the figure of the mirrors for the large Hawaiian telescopes and remove the normal errors in figure that result from mechanical grinding and polishing when the grinding tool overlaps the edge of the mirror. However, because of the dispersion of glass (variation in index of refraction with wavelength), a single lens element can only be optimized for a limited number of wavelengths, possibly just one. It will be out of focus for all other wavelengths. Mirrors differ in that reflection is essentially the same for all wavelengths, so a mirror figured to focus at, say, 400 nm wavelength (violet) will also accurately focus at 700 nm wavelength (red). -- Rodger Whitlock Victoria, British Columbia, Canada "To co-work is human, to cow-ork, bovine." - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org . ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept by MIMEsweeper for the presence of computer viruses. www.mimesweeper.com ********************************************************************** - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
