Why do dark jeans appear blue when photographed with a SLR camera? . . Ask for expert guidance~

Because you trust the camera's automatic exposure, the camera's light meter is based on 18% grayscale reflected light as the correct exposure in most shooting environments, but when you take photos of dark jeans (The reflectivity is lower than the grayscale of 18%) The camera metered the photo at 18% and got your current effect. The correct method is to do exposure compensation, and reduce the exposure by 2 stops (-2EV) When manually exposing, you need to narrow the aperture by two steps or shorten the shutter time by two steps based on the metering value given by the camera. If you are in a studio, you need to reduce the amount of light emitted by the flash by two steps or narrow the aperture by two steps. It should be noted that the aperture and shutter of most cameras today are adjusted in 1/3 increments.