All Tau2 cameras with lenses are factory-focused at the hyperfocal distance. A lens focus tool accessory is needed to refocus Tau2 cameras fitted with wide field of view lenses. The FLIR focus tool part number is 421-0037-00, price $40. A similar tool is offered by Cole-Palmer, PN T198342, also $40.
The lens cell has four indentations that correspond to the four pins on the inside of the focus tool. The focus tool has a hole in it by design to allow the user to see the image as the focus is being adjusted.
There are no specific focus instructions. The lens cell has an o-ring that compresses against the inside of the lens holder. Besides providing an environmental seal, this o-ring serves to keep the lens cell from moving, to the point where it seems very stiff/tight when the lens is re-focused. Some customers have been surprised with how much force it may take to turn the lens cell with the focus tool, but this is normal.
Use of a focus tool is also necessary to remove and replace a Tau2 lens cell. For this operation, it is recommended that the front of the camera be pointed down in order to minimize dust and/or other airborne particles from potentially settling on the sensor window.
FLIR Lens Focus Tool
Cole-Palmer Focus Tool