Precise focus is only possible at an exact distance from the lens at that distance, a point object will produce a point image. Focus point is on the first blocks column. Aperture settings are adjusted more frequently in still photography, where variations in depth of field are used to produce a variety of special effects.ĭepth of field for different values of aperture using 50 mm objective lens and full-frame DSLR camera. Motion pictures make only limited use of aperture 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. This observation contrasts with the common notion that 'focal length is twice as important to defocus as f/stop', which applies to a constant subject distance, as opposed to constant image size. For example, if the focal length is doubled, the subject distance is also doubled to keep the subject image size the same. This is evident from the DOF equation by noting that the ratio u/f is constant for constant image size. įor a given size of the subject's image in the focal plane, the same f-number on any focal length lens will give the same depth of field. Because the angles are shallow, the light rays are within the acceptable circle of confusion for a greater distance. Reducing the aperture diameter (increasing the f-number) increases the DOF because only the light travelling at shallower angles passes through the aperture.

Effect of lens aperture įor 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). If the focal length is altered to maintain the field of view, the change in focal length will counter the decrease of DOF from the smaller sensor and increase the depth of field (also by the crop factor). The resulting image however will have a different field of view. Because the circle of confusion is directly tied to the sensor size, decreasing the size of the sensor while holding focal length and aperture constant will decrease the depth of field (by the crop factor). Sensor size affects DOF in counterintuitive ways.

As a result, photos taken at extremely close range have a proportionally much smaller depth of field. Depth of field changes linearly with F-number and circle of confusion, but changes in proportion to the square of the focal length and the distance to the subject. Īs 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. The approximate depth of field can be given by:ĭOF ≈ 2 u 2 N c f 2 įor a given circle of confusion (c), focal length (f), f-number (N), and distance to subject (u). Limitations of depth of field can sometimes be overcome with various techniques and equipment. The depth of field can be determined by focal length, distance to subject, the acceptable circle of confusion size, and aperture. 'Acceptably sharp focus' is defined using a property called the ' circle of confusion'. Decreasing the aperture size ( 4) 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.įor 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. The points in focus ( 2) project points onto the image plane ( 5), but points at different distances ( 1 and 3) project blurred images, or circles of confusion.

Shallow depth of field f stop