Lumenera Lt-C1610, User Manual

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Teledyne Lumenera Lt Series Additional Reference Information • 123
7.2.4.3 Back Focal Variance when using any Filter
Inserting a filter between a lens and sensor changes the back 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 refraction of light through glass filter media.
sensor surface
(focal plane)
Focal Point with
filter is behind
sensor surface
Filter
Illustration: Change of Focal
Point with inserted filter
Incident Light
(from Lens)
In this example when a glass filter is inserted between the lens and the camera sensor, the focal
point is now about 1/3 of the filter thickness behind the sensor plane. Filters are typically specified
as 1mm thick.
For Lt Series models normally shipped without filters, when a filter is installed a fixed focus lens
requires a 1/3mm C-mount shim (spacer) added to move the lens focal point back to the sensor
surface. Such shims are available from filter and lens suppliers. Alternatively use a variable focus
lens and secure its focus ring after adjustment.