Piranha Color Camera User’s Manual 71
DALSA 03-032-10211-01 5/9/2007
Figure 10: Primary Points in a Lens System
SECOND PRINCIPAL POINT
SECOND FOCAL POINT
f
BACK FOCAL LENGTH.
LIGHT RAYS FROM LEFT
OBJECT
HEIGHT (h)
FIRST PRINCIPAL
POINT
OPTICAL AXIS
OBJECT
DISTANCE (OD)
IMAGE SIDEOBJECT SIDE
Magnification and Resolution
The magnification of a lens is the ratio of the image size to the object size:
m =
h '
h
—
where m is the magnification, h’ is the image height
(pixel size) and h is the object height (desired object
resolution size).
By similar triangles, the magnification is alternatively given by:
m=
f '
—
OD
These equations can be combined to give their most useful form:
h '
h
—
=
f '
–
OD
This is the governing equation for many object and
image plane parameters.
Example: An acquisition system has a 512 x 512 element, 10µm pixel pitch area scan
camera, a lens with an effective focal length of 45mm, and requires that 100μm in the
object space correspond to each pixel in the image sensor. Using the preceding equation,
the object distance must be 450mm (0.450m).
10 µm
100 µm
—=
45 mm
OD
OD=450 mm (0.450 m
—
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