On Wed, Apr 4, 2012 at 2:32 PM, George Sinos <[email protected]> wrote:
> If that's true people should be saying this "camera body back-focuses > with this lens" instead of the more commonly phrased "this lens has a > back-focus problem." I can't say I understand exactly why this is, but phase detection AF errors can apparently be caused by the body or the lens. I understand why a lens with spherical aberration could front/back focus when used at an aperture setting that's not the same as what the AF sensor is using (often f/5.6); I'm not sure if this would be significant. But there seems to be more to it than that: different copies of the same lens apparently will focus differently on the same body (see http://www.lensrentals.com/blog/2008/12/this-lens-is-soft-and-other-myths). Aside from the optics, at least in some systems, maybe all, AF lenses tell the camera body what the focus distance setting is. This, combined with the "how far out of focus is it" information from the AF sensor, lets the camera body calculate how much to adjust the focus. I would think that errors in this focus distance encoding would lead to multiple iterations before locking on focus, but not errors in the final locked focus point. Can anybody explain the origin of lens-related phase detection AF errors? -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
