This is something I learned to use that helps me evaluate certain aspects of what I am painting or drawing. If I am having trouble identifying the exact value or color of something, I found that isolating the area with a viewfinder helps me figure out how that area relates to another.
It is a simple tool as I just take a piece of cardboard and punch a hole in it. This creates a viewfinder that allows me to look at one area of the scene separated from the rest of the scene.
Today, we are going to look at how to use this type of viewfinder to examine the value of the local color.