Let me give a stab at this. This are the facts as far as I can tell:
- Inaccurate focus with several 50mm lenses on a K20D when assessed with a focus target
- These same lenses work fine on a K100D
- Same problem with "trap focus"
- Other manual focus lenses seem to work fine on the K20D
Here is what is not clear:
- What is being used to determine when the subject is in focus? Are you using image sharpness on the focus screen or are you relying on the focus confirm from the camera?
- If using focus confirm, where is the focus point set to?
- If using the focus screen, have you calibrated the viewfinder diopter adjustment to your eyes?
- What focus target are you using? Front/back focus strip?
If you are using the front/back focus target, I would suggest that you not bother. I may be wrong, but I don't believe that you can do a front/back focus adjust for any non-AF lenses on your K20D. (
Other members please correct me if this is wrong.)
Here is what I would suggest to test your focus:
- Calibrate the viewfinder diopter to your eye using a blank white wall. When the grid lines appear sharp, you are in business.
- Flat vertical target with printed lettering of varying size (cereal box?)
- Camera 1 meter from target (20X the focal length)
- Sensor plane parallel to target
- AF point set to center
- Adequate natural light
- Lens at maximum aperture
- Tripod
- No SR, 2 second shutter delay
- Do a best of 5 test using the viewfinder
- Repeat using the focus confirm (green hexagon lit)
- Evaluate the images primarily for center sharpness
My expectation are that the better results with the M 50/1.4 and the stock focus screen would be from the AF system. The stock focus screen sucks for fine focus at wider apertures. I would also expect somewhat soft results wide open regardless of focus method.
If you are able to reliably attain accurate focus using the viewfinder, but focus confirm fails using the same setup, there might be an issue with the default bias in the AF system. (Reset?)
Steve
(Seldom use focus confirm myself...have an aftermarket focus screen instead...)