Lens based autofocus inconsistencies

I have an FA lens which is focusing inconsistently.
It does it on both k10d and 20d. I have taken several shots on a tripod and it is inconsistent.

Could any body tell me what fault a lens could have to cause this problem? Dust, misalignment, focusing gears?

Any advice would be much appreciated.

Kiwao,

Which lens is it? I'm guessing the FA50 1.4 in your signature.

When you say 'inconsistent', what exactly are the inconsistencies? Is there a pattern to what is happening? Is it front-focus, back-focus, or both? How much is it out in either case?

What sort of settings were there on your cameras when this took place?

Those sorts of answers might help narrow down any problem.

In addition to the points mentioned above what was the subject matter, it would help of you could post some examples.

Thanks for the replies.
It is not in my signature. I bought a used 77ltd 2 months ago. Is this lens susceptible to focus shift?
WB daylight, various fstops and iso, centre point focus.

I looked through my photos to find some to post and looking at them again (he writes embarrassingly) it's probably my fault. I use centre point focusing and the images are at about infinity so I am probably misjudging the lack of contrast in the small object I am focusing on. I am sorting through them at this moment. I will post and so you can judge for yourself.

Images taken at or near to a lens's minimum focus distance are easier to use for diagnosing focus problems - not that ones at infinity are entirely useless, but they're less useful.

The only thing I can think off is that one time it approach the focus from the front and the other time from the back.
Since the field of focus is a certain distance wide the focus can be a little different but not by much though.

Thanks for the comments. At last I have put together some images.
image 8484- When the centre point was lined directly over the left red square it would not focus giving me the green flashing hexagon. Move it a little in any direction it focused and fired. Same with the image 8413 . I had my da70 with me so I tried that and no problem focusing but I suppose the lenses are too different to expect them to behave the same.


link

You are perhaps trying to focus on an area without any detail in it, the camera is then unable to focus. Pages 108 and 264 of the (English) user manual explain this.

Thanks for your comments

Yes you are probably right about most of the images. As I said myself I am probably misjudging lack of contrast in the central part of the image ie the boat on the lake- any detail is masked by the low contrast but I would have thought that a big red square with white text that completely fills the brackets of the center point mark would be enough detail for the autofocus to work and indeed it was enough detail for the autofocus to work with the da 70.

The da 70 autofocuses at f2.4 the 77 focuses at f1.8 so I would have thought that the 77 would be more consistently accurate.

Anyway I am so worried that I bought a dud that I am probably looking for faults that are not there.

Thanks again