Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 236)

Using the Eyepiece Shutter
Light that enters the viewfinder if you shoot without looking through it (in self-timer shooting,
bulb exposures, or remote switch shooting) may cause shots to be underexposed. To
prevent this, slide the eyepiece shutter lever as shown by the arrow to cover the viewfinder
eyepiece.
Note that covering the viewfinder eyepiece is not necessary in Live View shooting or movie
recording.
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