Yeah I can also do this, and I believe that's just your eyes accommodating. Your eyes usually do that automatically when looking at further away objects/things very close to your eyes (e.g. reading), and you're just doing it manually by forcing the muscle to tense up or relax. Some people can't do it as much, as your eyes naturally start to lose the ability to accommodate as you get older, hence why many more older people wear glasses compared to younger people, and different prescriptions are needed for different working distances. Sorry for the wall of text, i'm just obsessed with studying eyes ;-;