Your BSing the point. There is no tolerance level for common focus. Common focus means common focus period. While there is no standard there is a precise definition which you can hold to a higher standard if you wish. And you can test this easily. Point the camera lens at a bright star (3rd magnitude or brighter is best) and look at three things. 1. Determine if there is a purple or bluish halo around the image of the star. 2. Check to see if the color fringing shifts when inside the focus point. 3. Do the same for slightly outside of focus. After all this you can defocus the star inside and outside till you get a large out-of-focus sphere (hopefully). From this you can look at the diffraction rings to determine the precision of the optical correction in fraction of a wavelength of sodium light. Good telescopes like APO refractors and some Maksutov designs can reach 1/10th wave although 1/7th wave is very good. Bad is below 1/2 wave and 1/4th wave is just OK. All of this is hard to see in low power lenses but you notice nobody uses the word "APO" unless the lens is at least 200mm and generally even longer. Kent Gittings
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Tom Rittenhouse Sent: Friday, November 23, 2001 4:55 PM To: [EMAIL PROTECTED] Subject: Re: APO Lenses (was Re: Layers, Sharp Focus, and New-Fangled Color Film) Yes, but. Whether that common focus is within a tenth of an inch, a hundredth of an inch, or a thousandth of an inch makes a lot of difference. There is no standard saying which, or any other value; so manufactures make their own decisions about that, and whether the lens needs to be apochromatic at all magnifications or just one specific magnification. Furthermore, some manufacturers use "Apo" to mean better than our other lenses. So by that diffinition an "Apo" lens could just mean it is better than a cheap piece of molded plastic. As the man said, there are no standards. --graywolf ------------------------------------------------- The optimist's cup is half full, The pessimist's is half empty, The wise man enjoys his drink. ----- Original Message ----- From: Kent Gittings <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Friday, November 23, 2001 9:30 AM Subject: RE: APO Lenses (was Re: Layers, Sharp Focus, and New-Fangled Color Film) > Untrue. The definition is actually consistent because derivative of the word > base is 3 colors not all colors. What is different is whether the optical > makers want to call their lenses "APO" when only the 3 colors of the > definition are brought to a common focus or whether they want to only use it > when all colors are brought to a common focus. Definition doesn't change > only the standard of the optical maker. > Kent Gittings > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED]]On Behalf Of Bob Walkden > Sent: Wednesday, November 21, 2001 4:00 PM > To: Shel Belinkoff > Subject: Re: APO Lenses (was Re: Layers, Sharp Focus, and New-Fangled > Color Film) > > > Hi, > > it's my understanding that there is no (inter)national standard (eg > ISO, ANSI, BSi, DIN etc.) definition of the word 'apochromatic', therefore > the lens manufacturers can do some humpty-dumptying and make the word mean > whatever they want it to mean. > > --- > > Bob > > mailto:[EMAIL PROTECTED] > > Wednesday, November 21, 2001, 5:57:20 PM, you wrote: > > > Now this I'd like to hear more about. Can you elaborate on this point, > > specifically, how do manufacturers get away with labeling a lens as > > Apochromatic when it isn't? Is the term "APO" used to imply that a lens > > is apochromatic, when, in fact, it's just a model name given to the > > lens? Which lenses that are designated as "APO" lenses are not, in > > fact, apochromatic? Which are? Just a few examples would be > > appreciated. > > > Would a lens have to be designated as "apochromatic" to be apochromatic, > > as opposed to being labeled as "APO", which may not be apochromatic? > > > Mike Johnston wrote: > > >> and virtually all camera lenses labeled as "Apo," aren't. > - > 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 . - 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 .
