114
Technical Specifications
Genie Monochrome Series-GigE Vision Camera
Back Focal Variance when using a Filter
Inserting a filter between a lens and sensor (as when using a C-mount filter), changes the focal point of the lens
used. A variable focus lens simply needs to be adjusted, but in the case of a fixed focus lens, the changed focal point
needs correction. The following simplified illustration describes this but omits any discussion of the Optics, Physics,
and the math behind the behavior of light through different media.
sensor surface
(focal plane)
Lens
Focal Point with
filter is behind
sensor surface
(see text)
C-Mount
Filter
Illustration: Change of Focal Point with inserted C-mount filter
In this example using the Midwest Optical Systems C-mount NIR/UV filter, the image will be projected about 1/3
of the filter thickness behind the sensor plane. In order to compensate for this, the fixed focus C mount lens needs to
be unscrewed (counterclockwise) 1/3 of filter thickness or 1000 microns. This is approximately 1.25 turns. A spacer
would be preferred else the lens will need to be secured in the correct position. Alternatively use a variable focus
lens and secure its focus ring after adjustment.
In simplified equation form:
3
t
d
Where:
d
is the change (increase) in back focal distance due to the filter glass
higher index of refraction
t
is the thickness of the filter glass
