AF on K10D

Somebody might know. Recently my SMC 18-55mm pentax kit lens as being playing up a little. For some reason when I use it wide open at 18mm the AF system doesn't work but if I close it slitghtly all is well.But it doesn't do it all the time. Do I have a broken lens or is it something to do with the camera? Any comments welcome.

Perhaps start off by cleaning the contacts on the lens and the camera body to see if that clears the problem.

Cheers for that John will try it tonight with a dab of cantact cleaner.

the AF system struggles a bit at extreme wide angles, my 16-45 doesn't focus well at 16mm and I often have to zoom in a bit to focus then zoom out to compose.

This isn't new, I used to have an 18-28mm for my film camera and that was an absolute pig to focus too. I think it has to do with the large depth of field at wide angles, makes it hard to pinpoint the exact focus point.

Modern zooms don't hold the focus point very well when zooming, so it's really best to focus after zooming.

I've noticed this effect with the Sigma 10-20 at the wide end, but never with the 18-55, and nothing to do with aperture either.

I'm a bit puzzled about this. The camera focuses with the lens wide open regardless of what aperture the camera is about to set it to when the shutter opens.

Why would a different aperture set in the camera affect whether the AF works?

I'd like a bit more specifics about exactly how the AF doesn't work, in which circumstances, and how repeatably. Otherwise, in the absence of a bit more information, I'd be a teensy bit inclined to put it down to something else - bad light, perhaps, or poor contrast in the subject.

Does the AF not operate, or does it produce an out of focus picture? If the latter, it may be back or front focusing a bit, and it's only visible at the widest aperture.

More information required, methinks.

I wonder if the OP is using "wide" as in focal length and not aperture?

I guess its possible that the lens is a bit soft at 18mm, particularly with the wide open aperture used for AF, which could upset the AF. My K10D was particularly sensitive to this although not with the 18-55mm but the 50mm f1.4 and sometimes with my first DA*16-50mm.

Glyn