Originally posted by RiceHigh Physical positions of the lens mount and CCD cannot be changed, according to what Jebsen told me.
Only that the AF module can be adjusted physically and most of the settings of the AF can be re-calibrated and re-programmed.
Pentax's AF system has been found to be adaptable to the aperture number selected, i.e., different f-number selected will change the behavior of the AF system, see this example:
RiceHigh's Pentax Blog: DA 21 Mis-Focusing - A Design Problem?
So, if the problem only occurred after adjustment at the Pentax service centre, got your unit back to them and ask them to rectify it. Asking here will help you to know more but will surely not get your problem fixed! :-)
Hi Gentlemen,
Thanks for your great advises as well as the linked information (which I quickly read thro').
Basically, I think what happened to Ben DA 21 also happened to my Kit Mk2. Except mine was Front Focus before the repair service
In fact, for very long time, I had decided to used DC "P&S" instead of using DSLR what's that I knew optics system(s) could be very tricky to deal with: Slightly mis-alignment with a little bit error could already ruin the system's performance because optics required accuracy in "one wave-lenght" levels. I learned this while I did laser projects in polyU time. Simple setup is better than system built with lens + mirror(s) etc....
I eventually switched to DSLR because I really started to feel not satisfact with my 7M pixels P&S whereas the latest P&S with higher M-pix seemed did not have real improvement on IQ (In fact, IQ of most of the Higher MegaPixels P&S really stepped backwards I realized). Obviously, now I get more after entering the "P" world but I still did not get what I supposed to....Ok....
Yes, I can confirm the "German guy's finding": focus (focal lenght of a piece of lens) depends on F-number. I can explain this phenomeon using my limited knowledge learnt in high school times.....It is caused by "lens imperfection". A real lens cannot be perfect in real life....In fact, when the Aperature is widely open, the focus at the centre of a piece of lens can NEVER be concidely overlapping with the focus point obtained at the "rim" of the same lens (not matter how high quality of the lens produced). This resulted that the "a real focus" of a bundle of parallel light from infinite, caught by a wide open lens, is in fact "an area" or in fact "a 3D oval region" rather than "a spacial spot". An "AL" or "ED" lens improved a little but not all.
But the "problem" here also caused by (or mixed complicately by) the characteristic of the AF sensor...how it responds to those images produced by non-ideal lenses.....
What I could say now is that I have a bit of BAD LUCK... that I already find a problem with my kit mk2.....not even before I buy a second piece of lens....In fact, the problem really not purely caused by the lens but by the "combined" error of the lens + the body when put them together..... List them: design error + illed tolerances + manufacturer QC + lack of final fine-tune + combined unceratinty when puttng them together + .. ..
I will cantact Jebsun again and decide whether I will buy more lenses to play with...!