The entrance pupil is a real or virtual opening that delimits the bundles of rays that enter an optical system .
It is identical to the aperture diaphragm if it is in front of the imaging elements ( lenses or mirrors) in the direction of light . Otherwise, the entrance pupil is created as an image of the aperture diaphragm in that it is imaged into the object space by the elements in front of it .
Importance of the diameter of the entrance pupil
The diameter of the entrance pupil is a parameter of the light bundles involved in the optical imaging and thus a parameter for the transmitted luminance . Depending on the position of the object, the diameter of the entrance pupil is calculated differently.
Object in infinity
Object position variable
Furthermore, the diameter is included in the f-number k (commonly used in photo lenses where the object distance is variable), where it describes the beam limitation together with the image-side focal length of the system .
Object location close
In microscopes or in general in optical systems in which the object distance is very small, the diameter of the entrance pupil together with the object distance determines the opening angle and thus the numerical aperture via an angle function .
Importance of the position of the entrance pupil
Based on the position of the entrance pupil, a distinction is made between three perspectives or ray paths:
The entocentric perspective
This is what is commonly understood by perspective mapping. It is found in the human eye , photographic lenses, and many other optical devices.
The entocentric perspective is characterized by the fact that objects that are further away but of the same size are depicted smaller in the image plane than closer ones.
It occurs when the entrance pupil is behind the objects to be imaged in the imaging direction. (Note that the imaging direction and "viewing direction" are opposite.)
The telecentric perspective
In the telecentric perspective, a distinction is made between the telecentric beam path in the object and image space.
With the telecentric beam path in the object space , objects of the same size, one behind the other, are imaged in the image plane with the same imaging scale (i.e. the same size).
This is used, for example, in measuring lenses (measuring microscopes) in which the dimensions of objects at different object distances are to be compared.
This beam path arises when the entrance pupil is at infinity on the object side, that is, when the aperture stop is in the focal plane on the image side .
The telecentric beam path in the image space is the reversed counterpart to the telecentric beam path in the object space, that is, an object is displayed with the same size in different image planes.
This is used, for example, in large-format projections ( drive-in cinema ) in which a movement of the image plane (screen) parallel to the optical axis would lead to disruptive, local changes in the size of the image.
This beam path is created when the aperture diaphragm is in the focal point on the object side and is therefore identical to the entrance pupil. In this case, the exit pupil is imaged into infinity on the image side.
The hypercentric perspective
With this display, objects that are further away but of the same size are displayed larger in the image plane than closer ones. It occurs when the entrance pupil lies in front of the objects in the imaging direction.
In image processing , hypercentric lenses are used to view the outer surface of cylindrical objects (pill cans, threads, etc.) from the front of the cylinder.
In the ideal case, the object axis is located on the optical axis of the objective. In this case, circles with an object radius and a center point on the cylinder axis are mapped into concentric circles and screw threads are mapped onto a spiral.
The front lenses hypercentric lenses must be significantly larger than the objects being viewed.
Knowing the position of the entrance pupil is important for creating panoramas from multiple individual images. Only if the camera is swiveled around the entrance pupil as a pivot point between the recording of different, overlapping partial images, can these later be put together on the computer during image processing without parallax errors to form a panorama image .
In the case of objects that are only very far away or at the same distance, it does not necessarily matter if a point deviating from the entrance pupil center - e.g. B. around the center of the image or one of the two nodal points - is pivoted, however, with the simultaneous imaging of distant and nearby objects. This can later become noticeable when the images are put together in the form of so-called ghost images .
Nodal point adapter
Since the entrance pupil of most cameras is not above the tripod thread, special (and often expensive) setting slides or nodal point adapters are required for precise panoramic image recordings , which are attached between the tripod and the camera. They allow a lens and the camera to be pivoted around a freely selectable pivot point (within the mechanical limits of the adapter). In contrast, there are simple turntables or indexing disks, which allow even recordings through an even division of degrees when pivoting, but do not allow a free choice of the pivot point.
In addition to the rotation around the entrance pupil, nodal point adapters also allow the choice of a different pivot point - of course also one of the two nodal points . The term nodal point adapter may come from the fact that the term nodal point was previously incorrectly used for the center of the entrance pupil. In any case, the nodal points have no meaning in connection with panoramic photography.