Using The Eyepiece Cover - Canon EOS 100D Basic Instruction Manual

Remote Control Shooting
F Remote Switch RS-60E3 (Sold Separately)
Remote Switch RS-60E3 (sold separately) comes with an approx. 60
cm/2.0 ft cord. When connected to the camera's remote control
terminal, it can be pressed halfway and completely, just like the shutter
button.

Using the Eyepiece Cover

When you use the self-timer, bulb, or Remote Switch and do not look
through the viewfinder, stray light entering the viewfinder can cause the
image to look dark. To prevent this, use the eyepiece cover (p.29)
attached to the camera strap.
During Live View shooting and movie shooting, attaching the
eyepiece cover is unnecessary.
310
Detach the eyecup.
1
Push the bottom of the eyecup to
detach.
Attach the eyepiece cover.
2
Slide the eyepiece cover down into
the eyepiece groove to attach it.
After taking the picture, detach the
eyepiece cover and attach the
eyecup by sliding it down into the
eyepiece groove.