100mm is telephoto, yes. It depends on the size of the sensor, as it would in visible light cameras.
If you look at FLIR's F series, for example, a 100mm lens gives you an extremely narrow 4.6° horizontal field of view in the 320x240 model or 6.2° in the VGA model.
For comparison, using a camera with 1/3" sensor, a field of view of 6° would take a focal length of ~45mm.
Ethan, good job explaining.
Thermal imagers are not sized according to CCTV standards. Thermal cameras are "detectors" that provide an image. The major resolutions in security are 320x240 and640x480. A 100mm long lens on a 640x480 produces a 6 degree field-of-view where a 100mm long lens on a 320x240 produces a 4 degree field-of-view. There is a lot more to the concept but this is just a quick overview. To better understand distances, lenses and field-of-view FLIR offers a free tool www.flir.com/raven that allows you to design using various focal lengths and imagers.
A 100mm long lens on a 640x480 produces a 6 degree field-of-view where a 100mm long lens on a 320x240 produces a 4 degree field-of-view.
Isn't it necessary to know the pixel size/pitch as well, to determine the FOV angles?