Modern DSLR cameras use the Phase Detect autofocus method to determine the correct point at which to focus the lens. This method is fast and generally accurate, and operated by light travelling through the lens. When you half-press the shutter button on your DSLR while looking through the optical viewfinder, it’s Phase Detect autofocus that’s used.
"Snow White and the Seven Dwarfs was to be the first full-length cel animated feature in motion picture history, and as such Walt Disney had to fight to get the film produced."
