Optics basics: focus, circles, depth of field: concepts and calculations

First explain some concepts: 1, focus (focus) and the optical axis parallel rays into convex lens, ideal lens should be all rays accumulation in mind, then to the proliferation of the cone, the aggregation of all light, known as the focus. 2. circles (circle of confusion) before and after in focus, light began to gather and spread, the point of the image becomes fuzzy