Canon eos rebel t1i - Manual (Page 195)

195
Remote Control Shooting
If you take a picture without looking at the viewfinder, light entering the
eyepiece can throw off the exposure. To prevent this, use the eyepiece
cover (p.23) attached to the camera strap.
During Live View shooting and movie shooting, attaching the
eyepiece cover is unnecessary.
1
Remove the eyecup.
Push the bottom of the eyecup
upward.
2
Attaching the eyepiece cover.
Slide the eyepiece cover down into
the eyepiece groove to attach it.
Using the Eyepiece Cover