Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 258)

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When you take a picture without looking
through the viewfinder, such as when
you use the self-timer, bulb exposure, or
a remote switch, stray light entering the
viewfinder can cause the picture to look
dark (underexposed). To prevent this,
slide the eyepiece shutter lever as
shown by the arrow to cover the
viewfinder eyepiece.
You do not need to cover the viewfinder eyepiece during Live View
shooting or movie shooting.
You can connect the Remote Switch RS-80N3 (sold separately) or
Timer Remote Controller TC-80N3 (sold separately), or any EOS
accessory equipped with an N3-type terminal to the camera for
shooting (p.487).
To operate the accessory, refer to its Instruction Manual.
1
Open the terminal cover.
2
Connect the plug to the remote
control terminal.
Connect the plug as shown in the
illustration.
To disconnect the plug, grasp the
silver part and pull it out.
Using the Eyepiece Shutter
F
Using a Remote Switch