On 6 Jul 2005 at 23:00, Jens Bladt wrote: > I always thought that purple fringing was Chromatic Aberrations and is > caused by the fact, that glass will break different colours (wave lengths) > differently. APO-lenses are dealing with this, in order to assure that all > colors (wave lengths) from the same spot in the subject hits the sensor/film > in > the same spot, thus don't "seperate" - don't cause fringing.
CA is countered in longer lens generally by employing ED glasses not so much aspherics. APO is a technical designation (often abused) that basically suggests that a lens is corrected in order that three frequencies in the visible spectra will fall into focus on the same plane. The fact that the lens is APO doesn't stop saturation induced sensor bloom, when shooting with my APO macro lens on the *ist D I still get images with bloom yet in optimum light conditions at the same focus distance and aperture there would no CA. > The posted lens > test seem to indicate, that APO lenses don't cause much Chromatic Aberrations > and that "ordenary" lenses causes CA, especially if not stoped down. Aren't > APO > lenses equiped with aspheric lens elements? Not necessarily, ASPH lenses are mostly employed in WA designs to remedy geometric distortions, many lenses that employ ASPH elements still exhibit CA. Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
