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Effect of aperture on blur and DOF
The points in focus project points onto the image plane, but points at different distances project blurred images, or circles of confusion. Decreasing the aperture size reduces the size of the blur spots for points not in the focused plane, so that the blurring is imperceptible, and all points are within the DOF.
For Cameras that can only focus on one object distance at a time, Depth of field is the distance between the nearest and the farthest objects that are in acceptably sharp focus. ‘Acceptably sharp focus’ is defined using a property called the circle of confusion.
As distance or the size of the acceptable circle of confusion increases, the depth of field increases; however, increasing the size of the aperture or increasing the focal length reduces the depth of field. Depth of Field changes linearly with F-number and circle of confusion, but changes in proportional to the square of the focal length and the distance to the subject. As a result, photos taken at extremely close range have a proportionally much smaller depth of field.
Sensor size affects DOF only in that changing the sensor size on a camera requires changing the focal length to get the same picture. It is the change in focal length that then affects the DOF.
Effect of lens aperture
For a given subject framing and camera position, the DOF is controlled by the lens aperture diameter, which is usually specified as the f-number (the ratio of lens focal length to aperture diameter). Reducing the aperture diameter (increasing the f-number) increases the DOF because only the light travelling at shallower angles passes through the aperture. Because the angles are shallow, the light rays are within the acceptable circle of confusion for a greater distance.
Motion pictures make only limited use of this control; to produce a consistent image quality from shot to shot, cinematographers usually choose a single aperture setting for interiors and another for exteriors, and adjust exposure through the use of camera filters or light levels. Aperture settings are adjusted more frequently in still photography, where variations in depth of field are used to produce a variety of special effects.