How to get the basic light intensity according to the picture of an object shot by a camera under direct flash? Thank you.

My understanding: you can't see the intensity of the surrounding basic light from the photos. Because in modern photography, with the development of camera and flash technology, it is very convenient to control the live light and flash of flash separately to achieve different lighting effects. For example, when using high-speed synchronous flash, you can use calendering technology to take the environmental content outside the main body irradiated by flash into the same effect as at night, but this photo was taken in the sun. So you can't judge the basic light intensity when taking pictures according to the light intensity reflected in the photos.