Any ideas?? The autofocus on my D70 (nikkor 18-70mm) seems to be malfunctioning.. push the shutter button half way down and the focus ring revolves and always returns to 'infinity' .. is it something to do with the settings or is it a fault?? Have tried a re-set??
You are probably trying to focus on objects without a lot of contrast. It is not a malfunction, the sensor needs to focus on something that has a differing contrast. If you try to focus on a plain white wall or anything plain, it will "hunt" for focus.
When the lens is having trouble focusing, try putting the focus sensor on something that has a different color than the rest of the picture.