Calculating the focal length
The parameters you need
Calculating the focal length often leads to misunderstandings concerning the determination of the
parameters necessary for the calculation. In the following text, we offer some general advice on
how to avoid these misunderstandings. Please find more detailed information in the white papers
Lenses - Selection and setup and Basic optics.
Please note:
• It is the responsibility of an on-site engineer to select and apply lenses in the context of real
applications.
• All calculation methods are based on ideal lenses. The differences to commercial lenses are
negligible in practice. Wide-angle lenses, however, may vary considerably from the ideal.
Content
Parameter 1: Object size.................................................... 2
Parameter 2: Working distance.......................................... 3
Parameter 3: CCD format................................................... 4
Calculating the focal length................................................ 5
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Last update: October 2006
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All weights and dimensions are approximate.
Calculating the focal length - The parameters you need
Parameter 1: Object size
This parameter frequently leads to misunderstandings. This is mainly due to the fact that not only the
"object" itself is to be represented but also the surrounding area. Consider the following example:
Access control example:
In this case the "object under consideration" is a
human face. To compensate for variations of the
position we also take the surrounding area.
Thus, to calculate the focal length our object
size is:
• Object width = 40 cm
• Object height = 50 cm
Object width
Object height
Calculating the focal length
Objektive
- The parameters
- Auswahl und
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need
Parameter 2: Working distance
The working distance is the distance between the object and the lens' leading edge:
nce
Work
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Calculating the focal length - The parameters you need
Parameter 3: CCD format
The camera's specification sheet provides you with the CCD chips format (1/4", 1/3", 1/2", ...).
Here you see the The Imaging Source FireWire camera DFK 21AF04 as an example (please see
www.1394imaging.com):
CCD format
Calculating the focal length - The parameters you need
Calculating the focal length
In order to represent an object completely on the CCD chip,
we calculate the focal length for the object height and width.
The smaller value is our lenses focal length.
The CCD's format determines its height and width (see table
on the right). Please find detailed information concerning the
CCD format in the white paper Basic optics.
Focal length of the height =
Focal length of the width =
CCD
format
CCD
height
CCD
width
1/4"
2.4
3.2
1/3"
3.6
4.8
1/2"
4.8
6.4
2/3"
6.6
8.8
1"
9.6
12.8
[mm]
[mm]
Working distance * CCD height
Object height + CCD height
Working distance * CCD width
Obejct width + CCD width
Access control example (page 2):
• Object width = 40 cm
• Object height = 50 cm
• Working distance = 3 m
• CCD format = 1/4"
Focal lenght of the height =
Focal lengthof thewidth =
3000 * 2, 4
= 14, 3 mm
500 + 2, 4
3000 * 3, 2
= 23, 8 mm
400 + 3, 2
The white paper Lenses - Selection and setup shows how to select a commercial lens based on
this theoretical values.
Calculating the focal length - The parameters you need