Back Focal Variance When Using A Filter - Genie M640 User Manual

Back Focal Variance when using a Filter

Inserting a filter either in front of a lens or 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.
Lens
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:
t
d ≅
3
Genie Monochrome Series-GigE Vision Camera
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
sensor surface
C-Mount
(focal plane)
Filter
Focal Point with
filter is behind
sensor surface
(see text)
Technical Specifications • 107