Master Thesis - Fachbereich Informatik

40 CHAPTER 3. HARDWARE CONFIGURATION
(a) (b)
Figure 3.5: (a) Standard perspective lens. Closer objects appear larger in the image than
objects of equal size further away. (b) Telecentric lenses map objects of equal size to the same
image size independent of the depth within a certain range of distances. Images are taken
from Carl Zeiss AG (www.zeiss.de)
where Z0 can be seen as the lens-object distance also denoted as working distance in
the following. This gives a good estimate how large an object will appear on the image
plane at a given distance Z0 to the camera with a lens of focal length f.
Depth of Focus Following the thin lens camera model, only points at a defined distance
to the camera will be focused on the image plane. Points at shorter or further distance
appear blurred in the ideal model. In practice, however, points within some range of
distances are in acceptable focus [24]. This range is denoted as depth of focus or depth
of field. This is due to the finite size of each sensor element, since there is no difference
visible in the image if a point is focused on the image plane or not as long as it will not
spread over several pixels [18]. The depth of focus increases with a larger F-number [18].
Minimum Object Distance (MOD) All real lenses have a certain distance at which
points that lie closer to the camera can not be focused anymore. This has both mechanical
and physical reasons. The MOD value is important, since it determines the minimum
distance of the camera to the objects in an application.
AngleofView The angle of view is the maximum angle from which rays of light are
imaged to the camera sensor by the lens. Short focal length lenses have usually a wider
angle of view and therefore are also denoted as wide-angle lenses, while lenses with a larger
focal length have a narrower angle of view. The angle of view determines the field of view
of the camera at a given distance and a certain sensor size, this means what part of the
world is imaged onto the sensor array of the camera.
Commonly short focal lenses are used to capture images of a larger field of view for
example in video surveillance applications that have to cover a larger area. With respect
to machine vision applications, such lenses can also be used for close-up images at a
short camera-object distance. The amount of radial distortion increases with a shorter
focal length. The fish-eye lens is an extreme example for a very short focal length lens.
Increasing the focal length increases the magnification. Thus, even smaller objects at