What does the camera use to measure distance?

That's red eyes, used to promote pupil contraction.

It may also be focusing auxiliary light. The focus sensor is sensitive to red light with a specific wavelength. Using this wavelength of light to produce a spot with simple geometry can make the camera focus accurately and quickly in places with poor contrast or dark light, even on white walls.

Some cameras have ordinary white light.

No matter what kind of laser ranging, the basic principle is similar. After the pulse is emitted, the time difference between the emission and reception of the pulse echo is calculated, and then the distance is calculated by using the propagation speed of infrared ray or laser.

Infrared rangefinder has never been used, laser rangefinder has been used. When the interval is tens of meters, the accuracy can reach millimeter level. When disturbed by sunlight during the day, sometimes the reflected signal cannot be captured, and the test distance will be shortened.

Cameras can also measure distances, but they are not very accurate. It uses the information sent back by the lens and can only be used as a reference value.