Entrance pupil

, The entrance pupil is a real or a virtual aperture that limits the incident light to an optical system beam.

It is identical to the aperture stop, if this is in the direction of light in front of the imaging elements ( lenses or mirrors ). If the aperture has still imaging elements in front of him, ye by these elements generated in the object space image is defined as the entrance pupil and effective.

2.1 The entocentric perspective
2.2 The telecentric perspective
2.3 The hyper- centric perspective
2.4 Panoramic Photography 2.4.1 Nodalpunktadapter

Importance of the diameter of the entrance pupil

The diameter of the entrance pupil is a parameter of the light beams participating in the optical image, and thus a characteristic value for the transmitted luminance. Depending on the object position of the diameter of the entrance pupil will be charged differently.

Object at infinity

The diameter is specified directly as a parameter with telescopes, or generally in imaging systems, in which the object distance is infinite.

Property location variable

Furthermore, the diameter goes into the f-number k (commonly used in photographic lenses, in which the object distance is variable ), where he describes along with the image side focal length of the system, limiting the beam.

Property Location close

In microscopes, or in general, in optical systems in which the object distance is very small, determines the diameter of the entrance pupil with the object distance on a trigonometric function of the opening angle and therefore the numerical aperture.

Meaning the position of the entrance pupil

Due to the position of the entrance pupil, there are three perspectives or beam paths:

The entocentric perspective

This is the most commonly used perspective with which the human eye works, but also in photographic lenses, as well as many other optical devices is applied.

The entocentric perspective is characterized by the fact that more distant, but equally sized objects are mapped smaller in the image plane as a qualifier.

It occurs when the entrance pupil is located in the imaging direction behind the imaged objects.

The telecentric perspective

In the telecentric perspective, a distinction the telecentric beam path in the object and in the image space.

When telecentric beam path in the object space large, one behind the other objects in the image plane with the same image scale are mapped (ie, the same size ) is equal.

This takes place for example when measuring lenses ( measuring microscope ) application, in which the dimensions of objects to be compared in a variety of object distances.

This beam is produced when the entrance pupil is located in the object-side infinity, that is, when the aperture is in the image-side focal plane.

The telecentric beam path in the image space is the seitenvertauschte counterpart to the telecentric beam path in the object space, that is, an object is imaged the same size in different image planes.

This is used for example in large- scale projections ( drive-in theater ) application, in which a parallel to the optical axis of movement of the image plane ( screen) would lead to disturbing, local changes in size of the image.

This beam is formed when the aperture is in the object- side focal point, and thus is identical to the entrance pupil. In this case, the exit pupil is to the image side is infinite.

The hyper- centric perspective

In this representation are more remote, but equally represented large objects in the image plane larger than more. It occurs when the entrance pupil is located in the imaging direction from the objects.

In image processing, hyper- centric lenses are used to look at the lateral surfaces of cylindrical objects (tablets doses, thread, etc.) from the front side of the cylinder.

Here, the object axis is ideally located on the optical axis of the lens. Circles with the object, and the radius centered on the cylinder axis can be displayed in this case, in concentric circles, and screw threads are mapped to a spiral.

The front lenses hyper- centric lenses must be significantly larger than the objects under consideration.

See also: Diaphragm (optics), exit pupil telecentric lens

Panoramic Photography

Knowing the position of the entrance pupil is important for the creation of panoramas from multiple frames. Only when the camera between the consumption of different, overlapping sub-images is pivoted about the entrance pupil as a fulcrum, this can be put together later on the computer without any special measures or error in image processing to create a panoramic image.

In only very far or equidistant image objects it falls not necessarily important when a deviates from the aperture center point - is pivoted, the case of simultaneous imaging of distant and in - for example, around the image center or one of the two nodal points objects that are close by does. This becomes noticeable later when assembling the images in the form of so-called ghost images.

Nodalpunktadapter

Since the entrance pupil is not with most cameras on the tripod mount, special (and often very expensive) Focusing or nodal point adapter are required for precise panoramic images Images which are mounted between camera and tripod. They allow it to pivot a lens, including the camera to an arbitrary rotation ( within the mechanical limits of the adapter ). In contrast, there are simple turntable or partial disks, which, through a uniform degree of breakdown during pivoting uniform shots.

In addition to the rotation around the entrance pupil Nodalpunktadapter also allow the choice of a different pivot point - of course, one of the two nodal points. The term nodal point adapter is probably because that the term nodal point was formerly used incorrectly for the center of the entrance pupil. In the context of panoramic photography, the nodal points in any case have no significance.