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When using auto focus my lens will not focus lock on a subject - Why?
Most auto focus systems rely on contrast to archive focus Eg; If you focus on a blank wall the body will not be able to achieve focus consequently the body will drive the lens continually back and forth as it can not achieve focus - this is known as focal hunting.
If this occurs you can change the lens over to manual focus.
Fault with a lens - the auto focus sensor or motor may have failed. This will cause the lens focus not to drive to the correct position.
Fault with the camera - auto focus sensor or the auto focus sub-mirror may have failed. If the senor or mirror fail the camera can not determine point of focus.