f-number

What it means

The f-number is the focal length divided by the width of the lens opening. A 50 mm lens with a 50 mm opening is f/1.0. A smaller number means a wider opening for that focal length, so more energy reaches each pixel.

Most hunting thermals are f/1.0. A few run as fast as f/0.8 or as slow as f/1.2. Night vision housings and digital night vision lenses list f-numbers too, often between about f/1.0 and f/1.6.

What it changes in the field

For thermal, a faster lens (smaller number) gives the sensor a stronger signal, which helps in low-contrast weather such as fog, humidity or a warm evening. It is also one reason NETD figures don’t compare across brands: a maker can test sensitivity with a faster lens than the one it ships.

For night vision, f-number matters more. Tubes and digital sensors work with scarce light, and a lens that gathers more of it gives a brighter, less grainy picture on a dark night or under trees.

What to look for

• Compare f-numbers at similar focal lengths. A long lens with a low f-number is a large, heavy and expensive piece of glass or germanium.
• Small differences, such as f/1.0 versus f/1.1, are minor next to sensor resolution and focal length.
• Some spec sheets print the lens as one string, like “F50/1.4”. That usually means a 50 mm lens at f/1.4, but check whether the 50 is the focal length or the front opening.

How we use it on this site

We show the f-number next to the focal length in the Lens row. It is not used in our calculations.