Here is how to determine exposure by looking at the path of the light

We all know the exposure triangle. The exposure triangle deals with the relations between shutter speed, Aperture and ISO. Those three aspects are dealing with the way that the camera sees light.

To better understand light, we can look at the complete path light goes through from the second it leaves the light source (say a strobe), through hitting the object, reflecting the light into the lens , and going through the different lens and camera elements, until it hits the sensor (or film).

This is quite a journey, and it sheds some light (sorry) on many of the aspects that contribute to exposure.

[The Science of Exposure and Metering | Filmmaker IQ]