The Entrance Pupil
In a lens, the entrance pupil is the apparent position of the lens’ physical aperture as viewed from the front of the lens. When viewed from the rear of the lens it is the exit pupil.
Figure 1 shows a simple lens, the outermost ray from the object is refracted through the physical aperture. Hence, to an observer, the physical aperture position has been shifted, and this is the entrance pupil. Hence, when any lens or series of lenses is between an observer and the physical aperture, the physical aperture will appear to be shifted.
In figure 1, there is no lens between the image and the physical aperture, and therefore no position shift. Hence, in this case, the physical aperture is the exit pupil.
Figure 2 shows a compound lens. A compound lens consists of an arrangement of elements rather than just a single element as in the simple lens. In this case, the arrangement and number of elements is unimportant. The salient point is the outermost ray must be refracted through the physical aperture- in this example, located to the rear of the lens. However, to an observer, the physical aperture appears toward the front of the lens. This apparent position is the entrance pupil.
The Importance of the Entrance Pupil
Adjacent images are stitched together to form a panorama. To stitch adjacent images a degree of overlap is required. High-quality stitching cannot be achieved if the overlapping sections of adjacent images do not have the same perspective. This change in perspective is the introduction of a parallax error.
Figure 3a the relative positions of objects A and B do not change while pivoting around the entrance pupil- no parallax error is introduced. High-quality stitching can be achieved.
Figure 3b the relative positions of objects A and B do change when not pivoting around the entrance pupil- parallax error is introduced. High-quality stitching cannot be achieved.
If there are no foreground objects (A) then there can be no parallax error.
It has been shown that the entrance and exit pupils are the apparent positions of the physical aperture caused by the surrounding lens elements. The entrance pupil is viewable from the front, and the exit pupil is viewable from the rear.
To stitch adjacent images, the lens must be pivoted around the entrance pupil.
From this the following conclusions can be drawn:
- In panoramography knowing the position of the entrance pupil is essential.
- The entrance pupil position is related to the field of view (FOV).
- Wide angle lenses have an entrance pupil towards the front of the lens.
- Telephoto lenses have an entrance pupil toward the rear of the lens.
- Zoom lenses have a different entrance pupil for each zoom position.
- Fisheye lenses and ultra-wide lenses have an entrance pupil very close to the front of the lens.
- The focal length of a lens can change on focus distance, and it is called focus breathing. It affects both fixed and zoom lens and is generally not a problem. If the focal length changes the FOV changes; hence, the entrance pupil changes.
The following questions still need to be answered:
- How is the entrance pupil determined?
- What accuracy is required for the determination and positioning of the entrance pupil?
- How do you pivot the lens about the entrance pupil when shooting a panorama?