What makes Emma Stone's Eyes Twinkle? The etiology of "Twinkling Eyes" is a multifactorial and fascinating phenomenon.
The first, and by far the most important, component for a sparkle in the eye - is that of a smile, and it must be a sincere smile.
A "high sincerity quotient" smile must have eyelids which are partially closed - it's an absolute requirement. It's also a profoundly important distinction that all "partial eyelid closure" does not necessarily yield smiles - and the remainder of the face must always be taken into account. Recall Clint Eastwood in "The Good, The Bad and The Ugly". His acting expression was often that of mild to moderate anger.
When there is a momentary partial closure of the lids, this action squeezes the very thin layer of fluid on the surface of a healthy cornea and conjunctiva (the "tear film") onto a temporarily smaller surface - ergo this thin fluid becomes a bit "deeper" for a few moments. This extra fluid "depth" acts as a more efficient mirror and reflects light with greater efficiency - giving her eyes a "twinkling" effect.
After careful examination of the image below, you will see two light reflections in each of Emma's eyes - one on the cornea, near (over) the center of each pupil - and another just above the margin of each lower lid - where the tear film has a bit more depth - giving us an extra "sparkling" source.
Another requirement for a true, Joy-Happiness Smile is the short-lived, dynamic, concave-up furrow in the lower eyelids.
A Sincere smile must also have a relaxed (non-contracted) forehead. Indeed a forced and insincere smile is easily and often betrayed by an elevated and furrowed forehead.
Another requirement of a sincere smile is that if any teeth are showing (sometimes no teeth are showing during a sincere suppressed smile), they must be only the upper teeth (the exceptions being sincere smile-associated laughing, a high camera/visual angle or a downward tilting of the head/neck).
When sincerity is present, the cheek muscles will primarily vector upward - and not as much out to the sides. False smiles pull the corners of the mouth out laterally, rather than upward. Thus the mouths of fake smiles are often opened wider (left to right) than those of true smiles.
In this image, Emma Stone is outside - therefore her pupils are smaller with the bright sunlight. If she were inside, her pupils would be larger. Large pupils make a larger, darker, highly contrasting background for the light reflection - and make the twinkle more visible (e.g., a lighter color against a darker, larger background is much easier for other people to see).
Besides the relative darkness (which is one reason why dimly lit restaurants feel more romantic) - both affection (of a non-sexual nature) as well as sexual attraction are two other very common causes for larger pupils. Greater attraction equals larger pupils - and larger pupils leads to more sparkling.
Another cause of an increased or decreased "Twinkle Factor" is the angle of the light source. Without light there can be no twinkle - so to get a greater reflection, the light must be positioned behind the observer (e.g., if your partner is looking at you, the light source must be behind them [but not directly, lest it will be blocked by their head]).
Of course it the light is too bright, this will irritate the eyes as well as contract the pupils - which is one reason why candles engender eye twinkling (candles are dimmer than artificial light). Moreover, if a candle is used as a source of light - the constant changing amplitude of the flame (e.g., flickering) will amplify the tinkling effect - giving an extra dynamic quality to the eyes.
The level of love must always taken into account. Frida Kahlo once said, "Take a lover who looks at you like maybe you are magic." The presence or absence of love makes a portion of the "twinkle" subjective. Much like beauty, some of the twinkle depends on the other set of eyes - the eyes of the beholder.
Summary: In short, a "twinkle"/"sparkle" in the eye is influenced by...
1. The eye portion of a sincere smile (which always has partial eyelid closure and greater reflectivity)
2. The mouth portion of a sincere smile (aka "Duchenne" Smile) with upward vectored cheeks
3. A Relaxed Forehead
4. Medically Healthy Eyes
5. The type of light source
6. The angle of the light source
7. Larger vs. Smaller pupils (e.g., the level of ambient light)
8. The presence or absence of Affection and Love
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