Canon DC40 - Manual (Page 58)

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 About the AF-Assist Lamp:
While you press the PHOTO button halfway, if the subject is too dark the assist
lamp may light up briefly in order to enable the camcorder to focus more
accurately (AF-assist lamp). You can also set [AF AST LAMP] to [OFF] so the
assist lamp does not light up.
- The camcorder may not be able to focus even if the AF-assist lamp is on.
- The brightness of the AF-assist lamp could be an inconvenience. Consider
turning it off in public places such as restaurants or theaters.