Yesterday morning, you looked good. Yesterday evening, before you went out, you're pretty sure you looked real good. So who the hell is this schlub in the Facebook album from last night, tagged with your name?
It's a phenomenon nestled somewhere between universal annoyance and urban legend: People see something different in the mirror than they do in photographs. More often than not, the former is controlled, predictable and palatable, while the latter is an endless source of nasty little surprises.
So, why the disparity? The answer is complicated, but it boils down to this: Your eyes, your brain, your mirror and your camera are all conspiring to sabotage your body image.
It's the camera
The camera adds ten pounds! At a certain point, this obscure TV adage became folk wisdom. While this particular effect probably refers specifically to television, and in particular the distorting effect of the convex curvature of older TV sets, it seems to hold true for regular folks, sometimes in still pictures as well as video.
Cameras sensors may be absorbing the same photons as our eyes, but they're doing so through a complex lens that can actually change the way you look. Most cameras, from the dumpiest point-and-shoots to high-end DSLRs, ship with lenses capable of adjusting to wide, zoom-ed out perspective, and tight, zoomed-in views. At both extremes, the lens plays weird—and potentially ugli-fying tricks.
A wide angle lens does as its name suggests, capturing an image spread over a wide angle. The field of view in a wide-angle shot is wide—wider than that of your own eyes. In pulling this off, some lenses create a sort of fisheye effect, which can bloat subjects in the middle, and stretch those on the outside. This, however, is instantly recognizable, and probably won't cause too much anxiety. In other words, If the shot looks like a still from an episode of Jackass, you probably shouldn't let it figure into your self-image too much.
But there's a subtler effect of wide lenses called wide-angle distortion: Since the field of view is super-wide, objects close to the camera will seem large, while objects just a bit further away will seem very small. Here's a scene from Lock, Stock and Two Smoking Barrels that illustrates the effect, starting a 4:18. (NSFW, sorta.)
The net effect is an illusion of size, both width and height. Subtle, sure, but it's there.
Telephoto lenses are usually seen as more flattering, giving the impression that the subject is flattened, and slightly compressing the width of your foremost features, like your nose or breasts. So you might want to think twice before fleeing the pesky paparazzi and their fancy zoom lenses; it's the tourist with the pocket cam whose snaps will make you look fat on the Internet.
Lens distortion isn't the only way a camera can screw with your visage. Flash illuminates subjects harshly, turning elegant faces normally accented by soft shadows into a flat, shadowless, cadaveric horror shows.
Whether these effects are annoying or used to advantage, they mean that what you see in photos is different than what you see in the mirror.
It's the Mirror
I don't mean to imply that the camera is the only liar, here, because mirrors are just as guilty. For one, they flip your image. The You you're most familiar with, then, is actually an exact opposite of how you look to others. Granted, it's an intuitive reversal, so it doesn't bother us when we see it, but it implants a self-image that's intrinsically wrong.
On top of that, there's the problem of perspective. People stand close to mirrors, but see their whole selves. This provides a reasonable perspective, but a unique one: it's the perspective of a person standing near to you, eyes proportionately closer to your head than to your feet. This is the perspective of a partner in conversation, not a photographer. Looking a certain way from three feet away doesn't mean you'll look the same from 15.
The physics of lenses and mirrors offer solutions to specific problems, i.e. OH MY GOD SO THAT'S WHY MY WONDERFUL BUTT LOOKS SO FAT ON FILM! However, these explanations don't speak to a more relatable weirdness about photography. It's a feeling of uncanniness. It's a sense that something about the photographed self seems unquantifiably different than the mirrored self. It's in your head.
Think about the act of looking on a mirror. It's incredibly limited You pretty much need to be facing forward, or else you can't see. You will always be looking slightly down at the rest of your body. You will pose for yourself, to achieve the most flattering look. You will hide fat behind folds of clothes, or minimize a strange facial feature with a tilt of the head.
Other people, including photographers, don't see this version of you. They see a version that you are rarely privy to, and that can seem wildly foreign to our ingrained sensibilities. As Slate explains, it's a bit like how people hate their own voices on tape, doubly so because we know that those foreign, goofball intonations represent that way that everyone else hears us. In photos, we see ourselves in various states of motion, in different contortions and from uncaring, neutral perspectives. Lenses may distort, sure, but in a powerful way, these uncomfortable photographs are closer to reality than our carefully images in the mirror.