Could someone share info about how astronomers know about microlensing effects? I (educate) guess that we start by setting eyes to a specific star and then com

Could someone share info about how astronomers know about microlensing effects? I (educate) guess that we start by setting eyes to a specific star and then compared its image before and after X amount of days. Then, if there is a huge delta, then people would zoom into that star again to understand why there was a difference.