How to measure 18% neutral ash in photography?

You need a professional gray card. The camera is set to automatic white balance, so that people or models can hold the gray card outdoors, then use the gray card to shoot portraits, import the film into the computer, open the image with Camera RAW (a plug-in for Photoshop to process raw files, which needs to be downloaded by yourself), and click the gray card in the picture with the white balance tool in the toolbar. The software will automatically correct the color and skin color of the people. . . .

Every time you change the ambient light, you need to shoot the gray card first, and then shoot the film. . .