What does the telemeter function in watches measure?

The telemeter function in watches is designed to measure distance based on visual cues, specifically by calculating the distance to an event observed from a specific point, such as a lightning strike. It works by using the speed of sound, which is approximately 343 meters per second (or about 1,125 feet per second) in air. When you see an event, like a flash of lightning, you start a timer, and when you hear the resulting sound, you stop the timer. The time elapsed can then be used to determine how far away the event occurred.

This function is particularly useful in situations where sound takes time to travel to the observer, such as during thunderstorms, allowing users to estimate how far away the storm is based on the time interval between the visual observation and the sound.

The other options relate to different measurements that the telemeter does not focus on, making them incorrect in the context of this specific watch function.