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Circle of Confusion
Hyperfocal Distance

Photography (Basic) - Introduction to photography and other graphic techniques
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Figure 1-28.–Depth of field. When a lens is focused on an object at a certain distance, other objects, both closer and farther than the focus distance, form larger circles of confusion. When the film is placed at a point corresponding to the lens focus distance, a clear image is produced (fig. 1-28). When the film is nearer or farther away from the lens than the corresponding lens focus distance, the image becomes  blurred  because  of  the  larger  circles  of confusion  caused  by  the  intersection  of  light  rays  either in front of, or behind, the film plane. Table 1-3.–Permissible Circle of Confusion Is Dependent on Film Size 16mm 35mm 2 1/4 x 2 3/4" 4  x  5" 8  x  10" Film  Size 0.0010 0.002 0.004 0.006 0.012 Diameter  (inches) Another factor affecting the circle of confusion is lens aperture. Decreasing a lens opening narrows the light rays passed by the lens. The narrower these rays, the smaller the circles of confusion when the image is not in perfect focus. In practice, this means that a small lens opening is used to record, as clearly as possible, several objects at varying distances. Even when the rays from some objects do not intersect perfectly at the film plane,  the  circles  of  confusion  ahead  or  behind  the  film are negligible and still appear as a sharp image. The size of the permissible circle of confusion depends on the film format size and the manner in which the film will be used. Experience has shown that the permissible circle of confusion should not exceed about 1/1000 of the focal length of the lens. This is normal for the  film  size.  The  generally  accepted  permissible  circle of  confusion  diameters  are  given  in  table  1-3. Figure 1-29.–Depth of focus. The minimum circle of confusion of most lenses is sharp. Consequently, the distance that the focal plane small. Thus the focal plane can be moved slightly and can be moved forward or backward from the plane of yet retain an acceptable sharp image. However, as the sharp focus and continue to produce an image of distance of the movement is increased, the circle of acceptable  sharpness  is  termed  the  depth  of  focus.  This confusion  becomes  greater  and  the  image  becomes  less depth  is  always  within  the  camera  (fig.  1-29). 1-24







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