Scheimpflug Principle - Depth of Field

Depth of Field

When the lens and image planes are parallel, the depth of field (DoF) extends between parallel planes on either side of the plane of focus. When the Scheimpflug principle is employed, the DoF becomes wedge shaped (Merklinger 1996, 32; Tillmanns 1997, 71), with the apex of the wedge at the PoF rotation axis, as shown in Figure 5. The DoF is zero at the apex, remains shallow at the edge of the lens’s field of view, and increases with distance from the camera. The shallow DoF near the camera requires the PoF to be positioned carefully if near objects are to be rendered sharply.

On a plane parallel to the image plane, the DoF is equally distributed above and below the PoF; in Figure 5, the distances y_n and y_f on the plane VP are equal. This distribution can be helpful in determining the best position for the PoF; if a scene includes a distant tall feature, the best fit of the DoF to the scene often results from having the PoF pass through the vertical midpoint of that feature. The angular DoF, however, is not equally distributed about the PoF.

The distances y_n and y_f are given by (Merklinger 1996, 126)

where f is the lens focal length, v′ and u′ are the image and object distances parallel to the line of sight, u_h is the hyperfocal distance, and J is the distance from the center of the lens to the PoF rotation axis. By solving the image-side equation for tan ψ for v′ and substituting for v′ and u_h in the equation above, the values may be given equivalently by

where N is the lens f-number and c is the circle of confusion. At a large focus distance (equivalent to a large angle between the PoF and the image plane), v′ ≈ f, and (Merklinger 1996, 48)

or

Thus at the hyperfocal distance, the DoF on a plane parallel to the image plane extends a distance of J on either side of the PoF.

With some subjects, such as landscapes, the wedge-shaped DoF is a good fit to the scene, and satisfactory sharpness can often be achieved with a smaller lens f-number (larger aperture) than would be required if the PoF were parallel to the image plane.