One of the secrets of the old masters is capturing light.
It is also one of the main struggles of the aspiring artist. The goal of this article is to explain an important principle about how light works: the relationship between inclination and brightness. This will help you bring your work to life with a sense of light. And it will help steer clear of the three most common shading mistakes.
The part of an object that receives direct light is called the form light. For the purpose of this article, we will focus on this area and see what exactly is happening with the shading.
What we are concerned with, is the change of brightness within the form light area. As the form turns away from the light source, it gets darker. These different tonal values are called halftones. They are generally grouped into light halftones and dark halftones. You will see why that is below.
The brightest area of any surface is where it faces the light directly. As the surface turns away from the light source, it gets darker. Here is an example of a simple plane. It's noon and the sunlight is coming straight from above. Explore how the plane's brightness changes as the plane spins.
Your turn! Make a guess. What is roughly the relationship between the inclination of the plane and its brightness? Use your intuition. We'll check the results in a moment. 換你了! 猜看看，平面傾斜角度和它的亮度大概是甚麼關係?運用你的直覺，我們等一下看結果。
Fill in the table below.
Making sense of nature解釋自然
If your intuition about this is like that of most, you will have assumed a light falloff that is close to linear. (100%, 90%, 80%, 70%… etc.) It's a good guess, but this is exactly where most of us get blindsided. In the real world, things behave differently.
Here is what is really happening. Light falloff is not linear. Instead, it follows a nonlinear falloff. There are different ways of calculating this, but Johann Heinrich Lambert’s Cosine Emission Law is a popular method and a good approximation.