Focal length
The lens number in millimeters, like 35 mm or 50 mm. A longer lens puts more pixels on a distant animal but shows a narrower slice of ground.
What it means
Focal length is the distance, in millimeters, from the optical center of the lens to the sensor when the lens is focused far away. It is usually the number in a thermal’s model name: the “35” in a 35 mm unit. Common thermal lenses run from about 19 to 60 mm, with wide scanners as short as 10 mm or less and long-range scopes at 75 mm.
What it changes in the field
A longer lens magnifies. Double the focal length and a coyote covers twice as many pixels in each direction, so you can identify it about twice as far away. The trade is a narrower field of view: the same lens that reaches out also shows half as much ground, which makes scanning a big field slower.
Longer lenses are also bigger, heavier and more expensive. Thermal lenses are made from germanium, not glass, and cost climbs fast with size.
A rough pattern many hunters settle on: a shorter lens (19 to 35 mm) on a handheld scanner for finding animals, and a longer one (35 to 60 mm) on the rifle for confirming and shooting.
What to look for
Read focal length with pixel pitch and sensor resolution. Pitch divided by focal length is the angle each pixel covers, and that sets detail at distance. A 35 mm lens on a 12 µm sensor reaches about as far as a 50 mm lens on a 17 µm sensor.
On night vision, a “50 mm” figure is often the width of the front lens, not its focal length. See objective lens.
How we use it on this site
Focal length drives our pixels on target, identification distance and field width numbers. When a maker leaves it out, we work it out from the published field of view and sensor width. On every model that publishes both, the two agree to within 0.1 degree, so this is arithmetic on the maker’s own figure, not a guess.