A Basic Guide to Circle of Confusion in Photography

In photography, the circle of confusion (CoC) describes a point of light directed onto a camera’s focal plane by the lens. Depending on the camera’s aperture, depth of focus, and field of view, the diameter of this dot of light might be extremely narrow when it hits the camera sensor, or it might be wider. The wider the diameter of the circle of confusion, the blurrier the dot appears to the human eye. The circle of least confusion is the smallest blur spot a given camera lens can produce. The circle of least confusion varies from lens to lens.

The size of the circle of confusion affects the sharpness of a photographic image. A small circle of confusion indicates a very narrow spot of light that creates a tiny point on the plane of focus; this is associated with very sharp focus. A large circle of confusion indicates a wider beam creating a blur spot at the focus point; this is not conducive to a particularly sharp image, but it can create a strong bokeh effect.

Mathematically calculating the circle of confusion is challenging. Fortunately, even the best photographers do not need to manually calculate circles of confusion to operate manual SLR cameras or DSLR digital cameras. By simply adjusting focal length and f-stop numbers, you can adjust the circle of confusion on your image plane without performing a single mathematical calculation, thus achieving sharper focus.

Circle of confusion values do come in handy when you use depth of field charts (DoF charts). Photographers in the field use DoF charts to ascertain the hyperfocal distance of a particular lens. The numbers on a depth of field table for a given lens tell you the distance you should keep between your camera and your subject in order to achieve perfect focus.

You can find DoF charts online, but many lenses—from brands like Nikon, Canon, and Sony—actually have depth of field scales inscribed on their focus rings. By adhering to the rules of your DoF chart, you can achieve maximum visual acuity without doing a single CoC calculation. Just use the hyperfocal distance prescribed by your chart, and you'll be set.

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