Objective lens
The front lens that gathers heat or light. Its size and material set much of a device's price, weight and reach.
What it means
The objective is the lens at the front of the device, the end that faces the animal. It collects energy and focuses it onto the sensor or tube.
Thermal objectives are not glass. Ordinary glass blocks the long-wave heat a thermal sensor reads, so they are usually made of germanium, a costly material. That is why a bigger thermal lens adds so much to the price.
On thermal, the lens is usually described by its focal length: a “50 mm” thermal has a 50 mm focal length. On night vision, a “50 mm” figure often describes the width of the front lens (its aperture) instead. The two are different measurements, and spec sheets are not always clear which one they mean.
What it changes in the field
A larger objective lets a lens be both long and fast, so it can magnify while still gathering enough energy. That means more reach. It also means more weight up front, a taller mount to clear the barrel, and a bigger, more expensive lens cap to lose in the dark.
What to look for
- On thermal, compare focal length and f-number, not just the lens diameter printed on the box.
- On night vision, check whether a figure like “50 mm F1.2” is focal length or aperture. We record it as aperture unless the maker calls it focal length.
- Look for a lens cap that stays attached, and a focus ring you can turn with gloves on.
How we use it on this site
We record the focal length, f-number and, for night vision, the aperture as the maker states them. Only the focal length feeds our calculations.