Infrared A/F:  
The ability for this system to properly focus is greatly reduced when the unit 
is in a low light situation.  The reason is because the system depends on reflected 
infrared pulses to determine the distance between the lens and the subject.  
In a low light situation the subject matter is darker and absorbs a great deal of 
the infrared pulses.  So only a fraction of the beam is returned to process distance 
calculations.

Piezo A/F:
  This system also becomes confused when shooting a subject with lack of contrast 
differences, because the Piezo system uses changes of frequencies returned by different 
contrast levels on the subject.  Without changes in the frequencies, the Piezo circuit has 
no difference in inputs to sample against each other to properly calculate distance.

When experiencing any of the above A/F effects, remember that one, or a combination of 
situations may lead you to believe that there is a problem, but the unit may be completely 
within manufacturer's specifications.

Most consumer models are Piezo a/f systems.

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