Do not set it if you have an EOS Digital camera or the EOS Rebel T2/300 X . ... If "AF- assist beam OFF" is set with the Speedlite or camera, the AF- assist beam will not be emitted.
If "AF- assist beam 0FF" is set with the Speedlite or camera, the AF assist beam will not be emitted . With Type-B cameras, even if .Fn-05-0 is sat, E-TTL II / E-TTL autoflash will not work,