Just to refresh everyone's memory: > My defective DA* 16-50 is ready to go to Pentax to correct the fact that > it won't autofocus to infinity at 16 mm on the K20D. The K20D has to go > with it. > > Before sending them off I thought I would check the focus asjustment of > the DA* 50-135. I tested it at 135 mm, figuring that the most rigorous > test would be at that focal length because that's where the depth of > field is smallest. It turns out that an adjustment of -4 gives the > sharpest distance focus at that focal length. But then I noticed that > when I zoom the lens to 50 mm it now autofocuses beyond infinity, and > the images are not sharp. > > Arghhh! Okay, I guess the DA* 50-135 has to go to Pentax too to be > calibrated to the K20D. But this is disturbing and frustrating. I have > been concerned that it may be difficult or impossible to adjust the AF > of zoom lenses through the K20D--adjusting for correct focus at one > focal length could mean incorrect focus elsewhere. What I have seen so > far (with both DA* lenses) confirms that suspicion. >
Joe, I think you ran the test backwards. Depth of field is not the issue, depth of focus is. As focal length extends, depth of field decreases, but depth of focus increases. Try calibrating your 50-135 at 50mm, and then see how it does at the long end. I am starting to think that my long time eschewing of zoom lenses may not be such a bad thing though. William Robb ----- Can someone please explain to me why depth of focus increases at longer focal lengths? Anyway, I tested the lens again this morning at 50mm, running from -2 to +2 AF adjustments. If there are any differences, they are exceedingly small. At 200% magnification I have to conclude that at 50 mm the lens performs best at 0 focus adjustment. But this still leaves the lens misfocusing at 135 mm. If I correct for that it misfocuses at 50 mm. Joe -- 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.
