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A camera setting that overrides the automatic focusing and sets it to infinity so that landscapes and distant subjects come out sharp regardless of where the camera would automatically have focused. This is an important option to have when shooting though glass as the focusing system can easily be fooled.
A camera with active autofocusing uses an infrared beam to determine how far away the subject is. This is fine providing the subject isn't behind glass, because the beam will think the glass is the subject. The system is also flawed when the subject is beyond the range of the infrared, then the camera usually switches to an infinity mode.