What are the things that are photographed on people’s faces before shooting? What is the function?

That is a handheld light meter!

Compared with the in-camera light metering system, the handheld light meter can have more sensitive and accurate light metering performance. For example, it can measure 1/10 of the aperture of the first stop, and is more sensitive to low light. The measurement angle can also be very small, such as 1 degree, which is also unreachable by currently common cameras with "spot metering function".

In some situations, such as landscape, still life, product, and model photography, it is very inconvenient to move the camera to measure the light. Instead, leave the camera on the tripod and hold the light meter to measure, think, and judge the requirements. Much more calm.

The in-camera metering system can only measure reflected light, while a handheld light meter can also measure incident light and flash.