Pupil Constriction and Dilation

So the other week I bought some lens extensions for taking macro images as one of my hobbies is to take pictures of nature. I thought I’d give filming in HD ago and chose the eye as my subject because of it’s intricately beautiful design.

As you can see the video reveals details of the iris and pupil including their movements- constriction and dilation in this case. This didn’t happen to a friend’s eye that I initially filmed. I’m puzzled by this…

After some research and looking at other videos that people posted I came across things such as autonomic nervous system, galvanic response, adrenaline levels… Maybe the pupil is changing in size due to the blood circulation? However someone suggested it could just be the lights. Although additionally, I should point out that I am affected by myopia- essentially shortsightedness which might affect the constriction/dilation as well?

I’m still not sure what exactly causes this relatively fast paced change in size in the pupil & is wondering if many people have eyes that behave like this….