After getting the camera and Hypnotize Big Daddy tonic, find a Big Daddy and follow him until he walks up to one of the wall vents. If you haven't already saved or harvested the Little Sister, he will hit the wall and one will crawl out. Take as many pictures as you can until you can't get any more research, then use Hypnotize Big Daddy. The Little Sister will crawl back into the vent. After Hypnotize wears off, the Big Daddy will hit the wall again, bringing out another Little Sister. Repeat the process to unlock the trophy.