Enable Mirror Lockup on the Rebel T7i/800D
One component in the optical system of a dSLR camera like the Canon EOS Rebel T7i/800D is a mirror that moves when you press the shutter button. The vibration caused by the mirror movement can result in image blur when you use a very slow shutter speed, shoot with a long telephoto lens, or take extreme close-ups.
To eliminate this possibility, your camera offers mirror lockup, which delays the shutter release a little longer than normal so that the picture isn’t recorded until after the mirror movement is completed. Of course, you should also mount the camera on a tripod so that camera shake caused by handholding the camera doesn’t create blur.
You can take advantage of mirror-lockup shooting only in P, Tv, Av, or M exposure mode. Enable it through Custom Function 10, as shown here.
Mirror-lockup shooting requires a special picture-taking process: Press the shutter button once to lock up the mirror, release the button, and then press it all the way down again to take the picture.