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Khan Academy
Khan Academy: Algebra of Focus
In this video we develop a formula which tells us how big the circles of confusion are when an object is out of focus.
Khan Academy
Khan Academy: Waves and Optics: Object Image Height and Distance Relationship
Explores the relationship between the height of an object and the height of its image through a convex lens, and relates this to the ratio of their distances from the lens. [2:22]