Scheimpflug principle

The Scheimpflug rule ( after the Austrian officer and cartographer Theodor Scheimpflug, 1865-1911 ) states that the image, lens and focal plane intersect at an optical imaging in a common straight line.

The Scheimpflug condition is met, ie that the desired object plane is imaged with maximum sharpness when object, lens and image plane intersect in a common line. The sharpness level as designated object plane into clear focus can thus be an inclined plane when either the lens or the image plane can be tilted a camera. The English term tilt effect (tilt: tilt, tilt ) aptly describes this process as the German technical term focus strain on Scheimpflug principle, because the depth of field is not enlarged.

The limiting case that intersect the three levels in a common straight line, whose parallelism. He is considered the normal camera, in the lens and the image plane are arranged mutually parallel fix. Consequently, the focal plane intersects the optical axis at a right angle, what is commonly known as self-evident.

A lens - plane is considered, the principal plane of the lens. Most lenses have two principal planes, an object-side and image-side. The Scheimpflug rule is precisely that the focal plane with the object-side principal plane at the same distance from the axis of the lens cuts like the image plane with the image-side principal plane, and that both lines of intersection are parallel. Both lines of intersection are on the same side of the optical axis.

Explanation and application

The rule was formulated by Theodor Scheimpflug, 1907. It emphasizes the general validity of the lens equation

With a = object distance (measured from the object-side principal plane ), a '= image distance (measured from the image-side principal plane ) and f = focal length of the lens. This is insofar generic that it assigns to each object point the resulting sharp image point. If the object to be imaged plane is not perpendicular to the axis of the lens, the image plane is not perpendicular to, but wrong to the extent that it describe the Scheimpflug rule and the Scheimpflug condition.

In a conventional, simple camera, the image plane (the film ) and the lens plane parallel to one another. Consequently, the sharp imaged object plane is parallel to both. The common line of intersection is to imagine an infinite distance. If now a (flat ) object photographed, which is oblique to the plane of sharpness, this object is mapped completely sharp only in the area of the intersection of sharpness level and object level. This is for example the case when photographed from below a tall building close-up ( see photo of the Cologne Cathedral ).

When using a bellows device or a tilt lens as an accessory or a view camera with bellows to film and lens plane can be tilted against each other. Thus their common line of intersection of the infinite shifts in the vicinity of the optical axis. The Relevant focal plane is aligned with the line of intersection. You can for example be tilted backwards (horizontal intersecting line ) and coincide with the front plane of the tall building that you shoot from close range with obliquely upward camera (see the right in the drawing). This front is now ready despite different distance of its parts from top to bottom sharp. The main application area is architectural photography and the photography of historic buildings. This large-format high definition images are required. The recording format is already large and the small depth of field. Therefore, it is particularly important to maintain the Scheimpflug condition.

Converging lines and their equalization

Scheimpflug took aerial photographs, in order to produce maps. In his time there were no planes, the images were taken from a balloon. He had to make oblique views because the balloon was not sufficiently maneuverable to reach any mounting points. When necessary rectification of the aerial photographs he discovered the rule named after him and set his equalization unit (a type of magnification device) accordingly, so that the sharpness contained in the recording was not lost.

Photographs are always perspective pictures (central perspective), that is, that parallels the object in general are not imaged in parallel, vertical lines often appear as so-called converging lines. Before the proliferation of so-called shift lenses (or the use of technical cameras ) such perspective corrections were the same as of Scheimpflug photo lab done with an adjustable magnification device. Was only one level tilted heavily grayed out (usually the disk with the photo), was the lens to get sufficient sharpness.

Parallel lines that are located outside the center of the object remain parallel in the image when the object plane parallel to the permanent lens moved only laterally (English: Shifted; see left in the drawing). Characterized the angle of view is " pivoted " off center, the entire object plane is imaged to the same scale. This process has no relation to the Scheimpflug condition ( except that the limiting case is considered, in which the three planes are parallel to each other ), but is incorrect manner often associated with it. The feed is thereby made ​​that corrective lenses are usually both tilted and displaced: Tilt and shift lenses.

Also in special overhead projectors, the document is undistorted projected onto a mounted above the heads of the audience vertical screen by lateral displacement of the lens, particularly common with incorrect way of Scheimpflug principle is mentioned. The lens is moved horizontally toward the canvas. The template will project forward and upwards deflected by a mirror lens behind (or in ) and thrown from there with the same inclination to the horizontal to the higher mounted canvas. It occurs no keystone effect, because retained rectangles, not posted as harnesses. As a helix angle of between 8 ° and about 30 ° is selected.

"Double " Scheimpflug

Some publications mention the possible tilting around two axes the dual Scheimpflug as opposed to simply Scheimpflug where the focal plane is tilted only about the horizontal or the vertical axis. Geometrically, the dual Scheimpflug is a simple tilting about an oblique axis. The Scheimpflug rule makes no statement about the direction of the line of intersection between the three levels and therefore includes the dual Scheimpflug.