A Short Course Book
Using Your Digital Camera
A Guide To Great Photographs

Circles Of Confusion

Depth of field is a result of different parts of a scene coming into sharp focus at different points inside the camera.
  • Points in the scene that fall on the plane of critical focus are projected as points onto the sensor.
  • Since the light forming these points is cone shaped, any point in the scene in front of or behind the plane of critical focus is projected onto the sensor as a circle, not a point. Called circles of confusion, these circles increase in size the farther they are from the plane of critical focus. The point at which they expand from sharp points to out-of-focus circles defines the planes of near and far focus, between which lies the available depth of field.

When a large aperture is used (top), the cone of light is wide and circles of confusion get larger quickly, making depth of field shallower. Smaller apertures (bottom) have narrower cones of light so the circles get larger more slowly so depth of field is greater.

Although the smallest aperture gives the greatest depth of field, the sharpest apertures are usually one or two stops larger.

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