Canon EOS 5D Mark IV L-series Zoom Lens Kit Owner's Manual

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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 may cause the picture to look dark 
(underexposed). To prevent this, use the eyepiece cover (p.37) 
attached to the camera strap.
Note that attaching the eyepiece cover is not necessary in Live View 
shooting or movie shooting.
1
Detach the eyecup.
 
While grasping both sides of the 
eyecup, slide it upward to detach it.
2
Attach the eyepiece cover.
 
Slide the eyepiece cover down into 
the eyepiece groove to attach it.
 
After you finish shooting, detach the 
eyepiece cover and attach the 
eyecup.
Using the Eyepiece Cover