The two most common problems I get questions about are with lens flare and ghosting
Flare happens in a backlight situation. If the sun is too low in the sky (or you tilt your lens up) it can come into the lens and add to the exposure. Typically it creates these rings of light on your image. A lot of the time you get this white or blue haze all over your image and it kills a lot of the color. When you are outside the simple fix is to use a lens shade. When you are inside the fix is to make sure all windows are behind you and not behind your subject when taking the picture.
With ghosting you get two separate exposures that make up the total combined exposure of the image. Setting your camera to AV or TV mode allow your camera to take in this combined light. If dark outside or you are inside in low light conditions you pretty much can’t hand hold the camera. If you want to use either of these two modes with flash set the camera on a tripod to steady it. Also, I suggest you have your subject sit still for the photo. Having even the slightest movement will cause some motion blur.
if you recognize the problem image then Switch to P mode or Green Zone Mode. In P mode I suggest you set the ISO to Auto ISO. The camera will recognize when the flash is turned on and not allow ghosting by using a higher shutter speed. This will also make it harder to get lens flare as well.
Copy and video are from my online course “Secrets of using Canon E-TTL On-Camera Flash for Better Portraits”