It's all about light and vibration frequencies. We perceive light as a constant unit, but in reality it is a rapid oscillation. As an example, I can cite the light from fluorescent lamps, which is forbidden to use in kitchens precisely because of the oscillations - because it often happened that the chef cut himself with a knife while cutting (his attention was caught at a moment of darkness). "They" show themselves in the dark, and the camera, if it is taken at that moment of darkness, can capture "them".
You can ask professional cinematographers or photographers about this. A camera with classic film is more likely to capture such a moment. Digital cameras behave differently.
It is also necessary to realize that time is not linear, "they" can work with a time pocket. I write the truth, but people don't believe it because they don't understand how it works.
Plus "their" presence lowers the vibrational frequency, so empaths recognize "their" presence immediately.