It's not just longer lenses but faster ones with bigger front elements. The acceptable F-ratio for controlling dispersion without ED type glass goes up (slower) the larger the front element regardless of the F-ratio. In simple achromatic systems the acceptable F-ratio is 3" at F/15. But if the front element goes up to 4" the ratio has to be F/20 or slower, while for 5" it goes to F/27 or so. So the focus dispersion problem is significantly higher for a 600/4 lens than a 600/5.6 or a 600/8 lens because the front element is around 6" instead of 4" or 3". The steeper the light path the worse the problem (faster the F-ratio). Kent Gittings Kent
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Fred Sent: Wednesday, December 19, 2001 8:39 AM To: [EMAIL PROTECTED] Subject: Re: why do tele-lenses have ED glass > does anyone know why tele-lenses have ED glass, and why wide > angles do not? First: As I understand it, low dispersion glass can be used to help prevent color fringing (sometimes visible as a red or purple edge on a viewed object). Since the different wavelengths (colors) of light are refracted by ordinary glass at different angles, glass designed to prevent this dispersion of light rays is cleverly called "LD" (for Low Dispersion) glass or (if the marketing department has its way - <g>) "ED" (for Extra low Dispersion) glass. Then: Lenses with longer focal lengths are the ones that tend to disperse the red and purple ends of the spectrum the most, if not corrected (or minimized, actually). The weak point in my little explanation here is that I do not know why this is more of a problem for longer focal lengths than shorter. (Sorry.) I guess "a little knowledge is a dangerous thing"... So, my "quick and dirty" answer to your question, Frank, is that telephotos naturally produce more color fringing than wide-angles, so that they benefit more from the use of LD or ED glass, designed to minimize fringing. Just don't ask me to explain why it is the long lenses that produce the most color fringing - <g>. Fred - 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 .
