That applet is a great explanation of how phase detection AF works, but I don't think it explains lens-to-lens variation in AF.
On Wed, Apr 4, 2012 at 3:27 PM, George Sinos <gsi...@gmail.com> wrote: > OK, I found my answer, and some animated examples here. > <http://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html> > > The short story is that phase-detection measures the error and tells > the lens which direction and how far to move to get into the correct > position. It's faster, but depends on everything being calibrated. > Sort of like saying "go three feet to the east and you'll be there." > If you both have accurate rulers and compasses it will work fine. > > The contrast detection method used with live view is iterative and > keeps sending correction messages until the image is focused. Slower, > but more accurate. > > So it boils down to the fact that lens and body manufacturing > tolerances are wide enough that, for phase-detect focus to be spot-on, > each lens-body pair needs to be micro-calibrated. > > If you can live with the slower contrast-detection focusing of live > view, it will probably be more accurate. > > Anyway, that sheds light on the old "my copy of this lens isn't > focusing" statement. It's more like "this copy of the lens on this > copy of the body aren't a good match." > > Bottom line, now I think I understand why the simple calibration > method can work. > > gs > > George Sinos > -------------------- > gsi...@gmail.com > www.georgesphotos.net > plus.georgesinos.com > > > > On Wed, Apr 4, 2012 at 1:56 PM, Bryan Jacoby <bryan.jac...@gmail.com> wrote: >> On Wed, Apr 4, 2012 at 2:32 PM, George Sinos <gsi...@gmail.com> 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 >> PDML@pdml.net >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> to UNSUBSCRIBE from the PDML, please visit the link directly above and >> follow the directions. > > -- > PDML Pentax-Discuss Mail List > PDML@pdml.net > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
