Circles of confusion, CofC, blur circles
Circles of confusion, CofC, blur circles | ||
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A 'perfect' lens would transmit a point of light reflected from a subject, and focus it as a same-size point on the imaging surface. If the subject is flat, and parallel to the imaging surface, then all points reflected from the subject would be focused as points on the sensor. However, most of the subjects we photograph are not flat, and are at various angles relative to the imaging sensor. Light reflected from those parts of the subject lying on either side of the main plane of focus will be rendered as disks of varying size, called circles of confusion (CofC) or blur circles. Because human vision is not perfect, it will accept certain circle sizes as sharp, even though they are not points. The size of the circle that is considered sharp will depend on various factors such as viewing distance of the image and degree of enlargement. There is a general consensus that a circle of 1 30 mm (0.03 mm) will appear sharp. This is valid for 35 mm or full-frame imaging sensors, and takes into account an assumed size of final reproduction (usually 8 × 10 inches). A smaller sensor such as an APS-C size will need to be enlarged by a greater degree, and therefore the circle of confusion used to calculate DOF is smaller. The figure of 0.03 mm is divided by the crop factor of the sensor to give a circle of confusion for a particular sensor size. For example, a sensor with a crop factor of 1.5× will use a CofC of 0.03 divided by 1.5 (i.e., 0.2 mm). | ||