When using a flash on the EOS 6D, 5D Mark III or 1D X, photographers need to be aware of these settings:
Auto ISO normally is locked at ISO 400 when an EOS speedlite is attached and turned on
Auto ISO can be lowered down as far as 100, if over-exposure would occur in fill-flash situations in bright outdoor lighting
If an EX-series speedlite is swiveled for bounce, Auto ISO can extend from 400 or up to 1600 (depending on level of ambient lighting in the scene) and allow for an extra measure of E-TTL exposure when flash is scattered by bouncing