I usually force myself to smile in order to please the picture taker or the person who is bound to add the picture to their collection. I find it more efficient to fake a smile and avoid facing the bickering or disappointment that can come from a person whose conception is that a picture should always convey preternatural joyfulness. Most of the time though I am in a state of calm joy with an "inner-smile" which manifests externally discretely in the shape of my lips, with the hint of dimples and aloof but compassionate eyes. Therefore, in certain situations acting natural is fine, but being on a picture invariably makes me feel self-conscious.
I smile in pictures, even if I find it annoying. It’s just a habit. Perhaps I started doing it because I wanted it to be over faster. When you smile, the photographer complains less, and the shot is taken faster.
I don't often smile in pictures unless candid. it's because i feel rather shy/awkward when being photographed, so any expression ends up looking really tense and fake. I just try to relax my face and snap it fast. Pics basically always turn out more to my liking when i do this.
The first man to raise a fist is the man who's run out of ideas. H.G. WELLS
The first principle is that you must not fool yourself, and you are the easiest person to fool. FEYNMAN If this is monkey pee, you're on your own.SCULLY