DSLR stands for Digital single-lens reflex camera. It has a digital imaging sensor. In this kind of camera, the captured image can be viewed in the viewfinder when the shutter button is pressed. Its shown through the main lens rather than through a secondary lens, so the user knows what has been captured.
He was a German psychologist who is known for discovering the forgetting curve. According to this curve, the biggest decline in memory happens within 20 minutes, and then 1 hour.