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If the image that you see of the subjects isn't sharp enough, ensure the volume is sufficiently and evenly lit, and focus the camera so that you can see a clear image of the subjects throughout the volume (or within the part visible to the camera). To focus the camera, loosen the grub-screw above the lens and screw the entire lens in/out.
For information on how to adjust the brightness of the lights, see Set the brightness of the Vicon IR lights.