Use Aperture to Control What Is Sharp

Aperture is the size of the lens opening, and its numbers run backwards: a small f-number such as f/1.8 is a wide opening that lets in lots of light and leaves only a thin slice of the scene sharp, while f/11 or f/16 keeps far more in focus from front to back. That is why portraits are often shot wide open and landscapes stopped down. Background blur also grows as you move closer to the subject and the subject moves further from the background. Most phones have small fixed openings, so portrait mode creates the blur in software; the same distances still make it convincing. Exercise: in aperture priority (A or Av), photograph a friend a few metres in front of a hedge at your lowest f-number and at f/11, then step closer and repeat.