Thread on highlights! Have you ever noticed when looking at something that the highlight moves around when you move your head? Why does this happen? Why does it only happen with the highlight and not the rest of the light?
The only reason we can see is because light reflects off objects and into our eye. There are two types of reflection: Specular reflection, or the highlight, is when the path the light takes to your eye is equal and opposite the angle it took to get to the object, like a mirror! 
Since it's a mirror reflection of the light source, it will often reveal information about what is lighting the scene. If it is a light bulb, you’ll see a circular highlight, if it is a fluorescent light, you’ll see the long shape wrapping around the form like a curved mirror.
Highlights move when we do is because the angle to get to our eye needs to match the angle of the light hitting the object. It’s like bouncing a pool ball to go into the pocket, the angle into it will equal the angle at which it hit the table. When we move this angle changes.
The other kind, diffuse reflection, is when the light goes beneath the surface, bounces around, and then comes back out in a bunch of different directions. Because some of the light gets absorbed, what we finally see is a colored object regardless of where we look from.
Diffuse reflection is often misunderstood as only being the smoothness of the surface, this only determines how blurry the highlight is, not how mirror like it is. For example, even if you polish marble and make it perfectly smooth, it will never turn into a mirror!
Since metals don’t let any light inside the surface and have no diffuse component they are used to make mirrors. You can have a mirror with an uneven surface, thus a reflection that acts partially diffusely, but it is still not a diffuse material since no light goes inside!
How can we use this practically in our paintings? Skin shows both types of reflections at once. Here’s the secret: The highlights will dance down the form where two planes meet, the point at which the angle will reflect the light back into our eye. Use this to show form!
You can design these shapes in a way that is best to both make an interesting design compositionally as well as to show the structure of what you are painting.
The better your knowledge of the planes of the face, the more confidently you can place the highlights where the planes meet.
As the highlight gets further down and further from the light source, it will get darker, but it will never enter midtone territory. For more on how to choose the value of the diffuse and specular see my previous tutorial on the oreo cookie theory!
If you are interested in my other previous tutorials here are the links to those:
Composition:
Exposure:
Composition:
Exposure:
I also offer a mentorship if you would like personalized instruction and feedback! I love breaking down these confusing topics in a way that makes them usable while painting, that’s where the real fun comes in!
gumroad.com/l/YUuEd
gumroad.com/l/YUuEd
Artist credit: John Singer Sargent, Ivan Kramskoi
