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.
Answer:
A valley fold is one where you fold the paper into itself. The paper should form a V when you unfold.
A mountain fold is where the paper is folded behind itself.
