[Ian Buick]

Hi I am having problems with the autofocus of a 3 y.o. Pentax K20D + Tamron 18-250 Superzoom. Problems have begun in the past 3 weeks, and are becoming worse with time. Previous to this the camera + lens has been problem free.

At long focal lengths the camera will not autofocus at all, in bright sunlight and regardless of the object, whereas it autofocuses straight away on the same objects at short to medium local lengths under the same lighting conditions. In fact the autofocus hunting problem seems to be similar to that which some cameras have in very low-light conditions, but is occurring for images taken in the middle of the day, under bright sunny conditions. When this AF problem occurs the camera seems to be choosing very slow shutter speeds (in green and other modes) for such bright conditions The same problem occurs in normal and continuous AF mode. I not am unsure if this is a lens (currently don't have access to another lens to check) or a camera (AF motor?) issue, but would be grateful for any suggestions.

regards,

Ian

[mtansley]

Could it be the aperture in the lens that isn't opening enough? Can you carefully remove the lens and using the stop down lever on the lens see if the aperture is opening & closing smoothly or seems sticky. Also try turning the aperture ring on the lens and see again if the aperture is opening & closing smoothly. Less light enters at the higher zoom settings of the lens so if the diaphram isn't opening all the way perhaps that could cause a problem.
There is probably a way to see if the camera stop down lever is working properly but although I know what to look for, I wouldn't want to advise on that procedure. The lens lever is more rugged than the camera lever.
Anyone else feel free to jump in here and correct me if this couldn't be the case.
Michael

[Ian Buick]

Hi Michael, yes, it is the lens the problem. I tried borrowed another zoom lens today and there was no trouble with AF at any focal length. Thanks for your suggestion.

regards,

Ian