When does the circle of confusion become perceptible to our eyes? An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot.

At this viewing distance and print size, camera manufactures assume a circle of confusion is negligible if no larger than 0.01 inches (when enlarged). As a result, camera manufacturers use the 0.01 inch standard when providing lens depth of field markers (shown below for f/22 on a 50mm lens). In reality, a person with 20-20 vision or better can distinguish features 1/3 this size or smaller, and so the circle of confusion has to be even smaller than this to achieve acceptable sharpness throughout.

A different maximum circle of confusion also applies for each print size and viewing distance combination. In the earlier example of blurred dots, the circle of confusion is actually smaller than the resolution of your screen for the two dots on either side of the focal point, and so these are considered within the depth of field. Alternatively, the depth of field can be based on when the circle of confusion becomes larger than the size of your digital camera's pixels.

Note that depth of field only sets a maximum value for the circle of confusion, and does not describe what happens to regions once they become out of focus. These regions also called "bokeh," from Japanese (pronounced bo-ké). Two images with identical depth of field may have significantly different bokeh, as this depends on the shape of the lens diaphragm. In reality, the circle of confusion is usually not actually a circle, but is only approximated as such when it is very small. When it becomes large, most lenses will render it as a polygonal shape with 5-8 sides.

CONTROLLING DEPTH OF FIELD
Although print size and viewing distance are important factors which influence how large the circle of confusion appears to our eyes, aperture and focal distance are the two main factors that determine how big the circle of confusion will be on your camera's sensor. Larger apertures (smaller F-stop number) and closer focal distances produce a shallower depth of field. The following depth of field test was taken with the same focus distance and a 200 mm lens (320 mm field of view on a 35 mm camera), but with various apertures

Originally Posted by Xiorell Ok here's something My camera takes pretty good quality Macros, it's not an SLR mind. Problem is, setting the camera to "super macro" kills the flash. Using this setting, with the dial set to Auto, is fine. However, I get slower shutter speeds, not ideal for pics of snakes. Using this setting on either Manual, or Shutter priority (with fast shutter set), just gives me really dark/black shots. Other than going out, buying 10 halogen headlights from a monster truck and flooding the room with light - Would there be any other method of getting macros, with fast shutter speeds? Any settings I might be missing (High ISO has either little effect at 1/80 or higher shutter, or just has to much noise)