PanoTools:
Re: Re: entrance pupil - nodal point
Dan Slater 2004-Nov-11 07:16:54
Hopefully this will help clarify the entrance pupil / nodal point issues
as related to stitched panoramic imaging:
1. The center of perspective is the lens entrance pupil, which is the
virtual image of the aperture stop when viewed from the front side of
the lens.
2. The entrance pupil location is generally not at the nodal point
location. For most stitched image applications, the nodal point
locations are not important.
3. The entrance pupil is not a point but a complex shape for wide angle
and fisheye lenses. Entrance pupil aberration (the shape of the entrance
pupil surface) causes the perspective center to shift forward with
increasing angle from the optical axis in fisheye and wide angle lenses.
If you stop down a fisheye lens, look at it from the front and
illuminate the rear with a flashlight, you will see the entrance pupil
as a bright dot. As you look at it more from the side, you will see it
move forward toward the front surface of the glass. This is the reason
that you should measure the entrance pupil position at the angle where
you will be forming the image stitches. Equivalently, the lens viewpoint
is from a more forward point when looking sideways (edge of image) than
when looking straight ahead (center of image). This perspective center
shift is on the order of an inch or so for typical fisheye lenses.
4. The perspective center (entrance pupil ) may be inside or outside the
lens. A lens designer can position the entrance and exit pupil locations
where needed for a specific application. In fisheye and wide angle
lenses, the perspective center (entrance pupil) is generally toward the
front of the lens.
5. A telecentric lens has either or both the entrance pupil and exit
pupil at an infinite distance from the lens. In the case of object space
telecentricity (entrance pupil at infinity), the center of perspective
is at an infinite distance from the lens, meaning that the lens provides
an orthographic (perspective free) view. This type of lens is often used
in machine vision systems to maintain a constant magnification of the
object for any depth.
6. Lenses that are telecentric in the image space do not change the
field of view as they are focused at different distances. For panoramic
cameras, this would mean that the working focal length would be
completely independent of the focus distance. Another application of
image space telecentricity is to eliminate color fringing in 3 chip
color CCD cameras.
Dan Slater
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