This animation illustrates the conditions necessary to obtain a clear image by employing ray diagrams for a thin lens. A clear image can only be obtained if all of the rays (3 are shown here) coming from the same point B are focused at the same unique point.
This condition only exists at one location. Elsewhere, the image is blurred.
You can drag the screen to illustrate the way to focus an image.

Click and drag the screen behind the lens.