Well... First the boring stuff...
EVERY decent control system has a certain amount of hysteresis. Hysteresis is created when a *small* distance about "perfect" accuracy is designated as having sufficient accuracy for acceptable performance (this is often called "deadband") and wherein active control is suspended. This is done to prevent chatter or excessive "hunting". Chatter or excessive "hunting" greatly increases wear, is hard on mechanical equipment and often creates excessive noise and vibration. The autofocus in your camera is a type of control system because most folks greatly dislike vibration and noise in their cameras. Further, they would be greatly dismayed if the life of the bearing surfaces wore out in months instead of decades; therefore hysteresis is actually a good thing. You know that your AF system has built in hysteresis because the lens does not continue to hunt (forever) on a stationary due to sensor noise and continuous slight overshoots. What the boring stuff means... This means that your autofocus system will NEVER focus on EXACTLY the same spot every time because the AF will stop when it determines that focus error is within the deadband and therefore "good enough". Now DOF, in distance units, changes with several parameters. This also means that the deadband, in distance units, is different for every focal length, every maximum aperture and every focus distance. Autofocus is a convenience (and a great one at that), but it will never be a substitute for critical manual focusing to accurately and critically position your DOF about the subject. Some AF systems may be better than others or have different features, but this is true for *all* AF cameras, regardless of brand or model. Regards Bob... --------------------------------------------------------------- "I don't mind if you don't like my manners. I don't like them myself. They're pretty bad. I grieve over them long winter evenings." -- Philip Marlowe (Humphrey Bogart) From: "AlunFoto" <[EMAIL PROTECTED]> (Jostein) Well... Fellow FA*600/4 owner Øyvind Hopland and I had a joint session trying to calibrate our lenses. None of us got consistent results. When focussing on the same subject, an eagle perched on a rock some 50 meters below us, we got widely different optimal calibrations. I don't recall the exact numbers, but we were on opposite sides of zero. Then we tried a closer subject, a lichen covered rock some 6-7 meters away. And got a different, also divergent, set of numbers. I gave up there and then, because the puffins were beginning to arrive. Øyvind has tried again later, but afaik he hasn't been able to make any sense of it yet. I think there are other factors involved besides back- or front-focus. The MTF not very high, so telling the calibrations apart is more difficult than for, say, the DA*300/4. And camera shake is difficult to rule out completely. I would like to repeat the procedure with the camera and lens placed on a dampening surface, like a huge beanbag or something, to get a thoroughly vibration-free set of samples. As it stands, in the non-calibrated position, it has yielded a decent ratio of sufficiently sharp shots. 2008/8/15 John Whittingham <[EMAIL PROTECTED]>: > Hi Jostein > > Now I'm really curious/ > > John > ________________________________________ > From: [EMAIL PROTECTED] [EMAIL PROTECTED] On Behalf Of AlunFoto > [EMAIL PROTECTED] > > 2008/8/14 John Whittingham <[EMAIL PROTECTED]>: >> I really need to do the DA* 300/4 I would think adjustment would be >> more critical/beneficial with longer tele lenses, was there much >> adjustment required on the 600? > > I ended up turning it back to zero. > >> Was there a consireable improvement? > > No. -- 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.
