Master Thesis - Fachbereich Informatik

3.2. CAMERA SETUP 39
adjusting screws, etc.). The lens, however, is the most important factor that determines
thepropertiesoftheobjective.
The most important parameters to specify a lens include the focal length, F-number,
magnification, angle of view, depth of focus, minimum object distance, and finally the
price. In addition, lenses can have a number of aberrations as introduced before in Section
2.1.3. Lens manufacturers try to minimize for example chromatic or spherical aberrations,
but it is not possible to produce an completely aberration free lens in the general
case (e.g. for all wavelengths of light or angles). In practice, lenses are composed of
different layers of special glass. High precision is needed to produce high quality lenses,
thus, such lenses can be very expensive. There are different lens types available including
fix-focal and zoom lenses. While fix-focal length lenses, as the term indicates, have a fix
focal length, zoom lenses cover a range of different focal lengths. The actual focal length
can be adjusted manually or motorized. For machine vision applications fix-focal length
lenses are usually preferable [40]. If the conditions are highly constrained, the best suited
lens can be selected a priori.
This section should give a brief overview on the most important lens parameters and
motivate the selection of the lens used in this application.
Focal Length In the ideal thin lens camera model, the focal length is defined as the
distance between the lens and the focal point, i.e. the point where parallel rays entering
the lens intersect at the other side (see Figure 2.4). In practice, the focal length value
specified by the manufacturer depends on the lens model used (which is usually unknown)
and does not have to be accurate. In applications that require high accuracy, a camera
calibration step is important to determine the intrinsic parameters of the camera including
the effective focal length with respect to the underlying camera model.
F-number The F-number describes the relation of the focal length to the relative aperture
size such as [18]:
F = f
(3.3)
d
where d is the diameter of the aperture. Thus, the F-number is an indicator of the lightgathering
power of the lens. Typical values are 1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and
32 with a constant ratio of √ 2 between consecutive values. A smaller F-number indicates
more light can pass the lens and vice versa. Camera lenses are often specified by the
minimum and maximum F-number, also denoted as iris range.
Magnification In the weak perspective camera model (see Section 2.1.3), the ratio between
focal length and the average scene depth Z0 can be seen as magnification, i.e.
following Equations 2.3 the magnification m is expressed as [24]:
m = f
Z0
(3.4)