Some problems about photogrammetry

The exposure principle of the camera is to adjust the overall brightness of the viewfinder to 18% gray by setting the combination of aperture shutters. When the viewfinder is all white or all black, the combined exposure of the camera shutter aperture will definitely make the picture equal to 18% gray. Photographing against a black wall will turn black, depending on whether your exposure ev is set to 0 and whether the light is sufficient.