It's shootin' day, and you just set up your camera so you can record yourself talking. It goes well, for the most part, and you eventually get all your lines sounding pretty good.
And then you go back and look at the footage, and realize... the whole thing was slightly out of focus.
Or, at best, parts of it are in focus, but a ton of really important information got lost when the camera decided to let your face turn blurry.
So, as you begrudgingly go back to reshoot, realizing that you probably won't have time for lunch at this point, you wonder: how can you be sure you're in focus when you're shooting by yourself?
Well, assuming you can't pull somebody aside to make sure your shot looks good, you can do the next best thing: trick your camera.