As I've said before here and elsewhere, I don't think the focus indication for use with older, manual focus lenses is particularly accurate. It's surely not accurate enough for a 50mm f/1.4 lens when wide open to be the sole discriminator of obtaining accurate critical focus.
If the image on the focusing screen with a 135mm f/3.5 *is* critically focused and the resulting exposure is not, then there is an alignment problem between the focusing screen and the sensor plane. This is not unheard of, it happens on many reflex cameras, and it is usually corrected by shimming the focusing screen to achieve accurate alignment. Of course, the big question is "Is the image on the focusing screen critically focused?" The standard way to test this is to use a locked down camera and focusing target with a high-magnification viewfinder accessory to determine that the image is critically focused on the focusing screen and a target where the focus variance is easily discernible. It sounds like you're trying to achieve this with your test ... which means that if the camera is consistently off, through a few tens of focusing attempts at a few different focusing distances, then screen to sensor alignment may well be the problem. If it is, the best way to have it adjusted is to have a technician with the right equipment do it. On Mon, Jan 10, 2011 at 7:23 AM, Eric Weir <[email protected]> wrote: > > On Dec 29, 2010, at 12:47 PM, steve harley wrote: > >> On 2010-12-29 08:30 , Eric Weir wrote: >>> >>> On Dec 27, 2010, at 6:24 PM, steve harley wrote: >>>> 3) you've got a problem with the focus and/or the lens >> >>> I wonder about lenses. I haven't attempted to evaluate any of them in this >>> connection -- and how would I know, in any given instance, that the problem >>> was with the lens? -- but I've tried to pick lenses that have a reputation >>> for producing sharp images and avoiding those that do not. My collection: A >>> 28/2.8, M 50/1.7, A 50/2.0, M 100/4.0 macro, 2 M 135/3.5s, and one A >>> 70-210/4.0. >> >> well, it could be the lens, but if the image appears very sharp in the >> viewfinder, but comes out blurry, your camera may be back- or front-focusing >> (viewfinder not properly coordinated with the actual focus on the sensor); >> some simple tests on a tripod can confirm that (the classic is to prop up a >> yardstick at 45 degrees and focus on the 18" mark) > > As mentioned before, I'm not experiencing a lot of success getting sharply > focused images. [Though I have a shot of a mural on the end of a row of row > houses in Pittsburgh shot from a moving car that's among the sharpest I've > gotten.] Finally got around to doing the test Steve suggested. Used an M > 135/3.5 lens wide open on a tripod about six or seven feet from the > yardstick. Focusing on the 18" mark both the 18" and 16" were in focus. > > That fact makes me wonder if this was an effective test of whether my camera > [*ist DS] is front- or back-focusing. If it were not, if it were focusing > properly, shouldn't the image appear out of focus as you move away from the > focus point? > > This has me thinking about spit image focus screens again, though I am > reluctant to go that route until I'm certain that all options that don't > involve aids have been exhausted. > > Need to continue monitoring it to be sure, but having turned the view finder > focus indicator back on my sense when checking it is that images come into > focus for me a little ahead of the "in focus" focus indicator. > > ------------------------------------------------------------------------------------------ > Eric Weir > Decatur, GA USA > [email protected] > > > > > > -- > 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. > -- Godfrey godfreydigiorgi.posterous.com -- 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.
