Nothing like a jarring pair of glowing red eyeballs to ruin the perfect shot. And while it's a simple enough correction, SciShow explains why it happens in the first place and what you can do to prevent it. As the video explains, the effect has to do with your eyes dilating. When the lighting around you is dark, the muscles in your iris make your pupils wider to let in more light. That way, you can see stuff. When your eyes glow red in photos, it's often because you're taking the photo in a dark room, and here's what happens to your eyes:
...if there's suddenly a bright camera flash, all that light floods into your eyes before your iris muscles have time to contract. Some of it might reflect off the blood vessels in the back of your eye, and shows up as a glowing red light to ruin that group photo you were trying to take.
Some cameras will actually flash a couple of times before taking the photo to give your eyes a chance to adjust to the light. SciShow suggests brightening the room before a photo, too, to make sure that your pupils aren't wide to begin with. Otherwise, avoid looking directly at the camera lens.
They offer some additional info in the video, along with some tips on how to tell if red-eye photos might actually signify eye problems. Check it out above and then head to their YouTube channel at the link below.
Why Do My Eyes Glow Red in Photos? [SciShow (YouTube)]