On 31/01/07, Brian Dunn <[EMAIL PROTECTED]> wrote: > > I received a 21 Limited. Center only autofocus always greatly back focuses on > both ist-DS cameras, more on one than on the other. The 45 degree focus > chart showed the 21 Limited back focusing almost off the far end of the > paper.
How does it focus on objects parallel to the sensor plane, I assume that you have pre-selected the central focus spot? > I tested other lenses and found that they all back focus when set to wide > angles. The amount seems to be proportional to the angle. For example, you > could set a 16-45 on a tripod, aim at something 7 feet away, zoom in to 45mm, > focus, then zoom out and focus again. The more you zoom out, the more the > autofocus changes. Focus is very much harder to judge when using extreme wide angle lenses and that goes for the AF system and by eye. Also system focus adjustment is very much more critical as any lenses angle of coverage is increased, lens to sensor plane distance has to be more precise as the depth of focus becomes smaller with focal length ant any given aperture (from recollection). My 16-45 doesn't maintain accurate focus as it's zoomed, and I know that other owners here have reported the same. > Has anyone here tried the adjustments under the base plate? No, but there was another discussion that I read that indicated that some AF systems also employ a firmware error offset table that's determined during the factory AF system calibration. whether this is actually the case or not on your particular Pentax body I'm not sure but if you're prepared to send it to Pentax to be aligned I guess a tweaking a few screws will add little to the cost of the AF adjustment procedure in the case that it goes wrong. -- Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://home.swiftdsl.com.au/~distudio//publications/ Pentax user since 1986, PDMLer since 1998 -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net
