Pupil Aperture Integration 

Pupils adapt to lighting conditions and basically operate similar to a camera’s aperture. A diagram below on aperture size vs. light intake:

Bright lighting conditions: small “aperture”/pupil size

Dark lighting conditions: large “aperture”/pupil size

A closer look:

To see a subtle pupil size shift in action by changing only lighting conditions, I thought I would find an example in a long-take music video. I found it in “WALLS” by Kings of Leon.

A closer look:

Pupil Emotion Animation

Pupil size can change with more than just lighting conditions. Their size changes with age:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5006796/

And in reaction to positive and negative emotions:

https://www.nature.com/articles/s41598-018-23265-x

Previous studies report that emotionally arousing stimuli, both auditory and visual, trigger bigger increases in pupil size than emotionally neutral stimuli. Within emotional stimuli, some authors found that negatively valenced stimuli (e.g., crying) trigger larger pupil dilations than positive stimuli (e.g., laughter), whereas others found that both positive and negative stimuli could generate equally large pupil responses

These concepts are related to animation and not so much integration, but can add a level of realism. How have films used this?

It took me serval viewings of this scene to notice his dilation. Below are tight closeups when characters don’t have limbs to act with making use of dilation to express emotions.