The closest I could find was this:

https://graphics.stanford.edu/papers/remote-pupillometry/

It seems that there are reasonably large changes due to thinking about something, but not due to just looking unless your camera can detect 0.01 mm changes in diameter, which maybe tbihl's could be that seems unreasonable for a smartphone camera.

I would guess tbihl has never tried to get iris recognition working on a cost-constrained consumer device at a huge scale. They might not think it is so trivial then.

It's true that I've only tried iris recognition in dedicated setups. Sadly, I can't speak to whether this test is within the abilities of a standard smartphone camera without any macro lens.

Saccades is not what I was referring to. 'Micro-dilations' was my attempt to describe the phenomenon. After some searching, I believe it's called hippus (maybe specifically 'physiological' hippus, to distinguish from pathological) or ' pupillary athetosis'.

A similar concept that I remember from university are saccades (minimal, involuntary eye movements).

Yes but those are well-known and visible to the naked eye. I've never heard of these 'micro-dilations' and Google didn't show anything.

'hippus', specifically 'physiological hippus', or ' pupillary athetosis', seem to be the correct terms for what I was describing as 'micro-dilations'. Saccades, as I understand it, is rotational shift of focus by 1-2 degrees, rather than repeated contraction/relaxation of the pupil.

I think you're thinking of microsaccades.

Saccades in general are just rapid, ballistic eye movements: they don't have to be small and they're often voluntary. You often make saccades without explicitly thinking about it, but you can countermand these and hold your eyes still when you want to.