Canon EOS 6D Instruction Manual page 360

Troubleshooting Guide
ISO 100 cannot be set. ISO speed expansion cannot be selected.
If [z4: Highlight tone priority] is set to [Enable], the settable
ISO speed range will be ISO 200-25600 (or up to ISO 12800 for
movie shooting). Even if you expand the settable ISO speed range
in [ISO speed range], you cannot set expanded ISO speeds (L, H,
H1, H2). When [z4: Highlight tone priority] is set to [Disable],
ISO 100/125/160 can be set (p.130).
The Auto Lighting Optimizer cannot be set.
If [z4: Highlight tone priority] is set to [Enable], the Auto
Lighting Optimizer cannot be set. When [z4: Highlight tone
priority] is set to [Disable], then the Auto Lighting Optimizer can
be set (p.130).
Even though I set a decreased exposure compensation,
the image comes out bright.
Set [z3: Auto Lighting Optimizer] to [Disable]. When
[Standard/Low/High] is set, even if you set a decreased exposure
compensation or flash exposure compensation, the image may
come out bright (p.151).
The multiple-exposure image is shot in 1 quality.
When the image-recording quality is set to 41 or 61, the
multiple-exposure image will be recorded in 1 quality (p.164).
When I use the <f> mode with flash, the shutter speed
becomes slow.
If you shoot at night when the background is dark, the shutter
speed becomes slow automatically (slow-sync shooting) so that
both the subject and background are properly exposed. To prevent
a slow shutter speed, under [z2: External Speedlite control],
set [Flash sync. speed in Av mode] to [1/180-1/60 sec. auto] or
[1/180 sec. (fixed)] (p.173).
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