White plus black minus

White plus black minus is something you should pay attention to when shooting some objects with abnormal colors.

For example, "white plus" means you take a blank sheet of paper.

Because of the working principle of exposure meter.

The given value is the EV value measured under the standard of reducing white paper to 18 degree gray scale.

If you want to restore the whiteness of white paper, you must increase the exposure.

The same is true of "black reduction"

Take a black piece of paper, and the value given by the exposure meter is 18 gray.