Focus (optics) facts for kids
In reality the focus is never a point but a small spot called the blur circle.
An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged.
Images for kids
Focal blur is simulated in this computer generated image of glasses, which was rendered in POV-Ray.
Focus (optics) Facts for Kids. Kiddle Encyclopedia.