Lighting or shading a face is about understandig the form. Understanding the form and moving the light source wherever you want.
Here is a geometric form of a face, skeleton tutorial may be helpful at this point (I drop its link at the end). Shortly I will tell where to shade; which is both sides of eyes, around eyes (preserve a tiny circular area for a few tones lighter under the tail of eyebrow), under lower lip, below cheeks, down part of chin and under chin, under nose and cupid’s bow above upper lip. You may find more about detailed shading in my previous tutorials of eyes, nose and lips.
I prepared 4 faces according to different light sources:
Once you got the logic, then you can move the light source like how you want.
The low quality of my drawing’s photos is the proof that I gotta do something with my camera. Photos will be updated.
For this study I tried to draw more or less the same face, 4 of them. Not to lose midtones I didn’t make high contrast/totally darks.
1- Light comes from side somewhere up, so one side of the face is lighter and on the other side shadow’s angle depends on the light source’s direction. The way face form catches the light is important. If you consider that the form is not flat, the lighter part of the face will still have some tones of shade, but they are going to be lighter/smooth in the compare to the other(dark) side.
2- The second example is used really a lot I saw, which is a quite cool thing, is like light comes from back and so the face is dark; so is whole hair and you can still see some hair as single lines around head-I did with a hard pencil you can see in the drawing. In this example the darkest area on face is the middle part, sides have a little of the light.
This is a funny fact to me that where I have an example from, is a video clip of Queensryche:
I didn’t use that strong light-shadow contrast like in the image above. As you see the middle part of his face is in the dark, but his nose catches some light partly, and this strand on forehead stands forward, so catches some light too, as well.
3- That one might be what you need if you are planning to draw or paint an interrogation scene :p In the third example, light source is in the up middle. That causes equal amount of shadows on the sides of the face.
You are going to shade eyelids, under nose/above upper lip and the cupid’s bow, under lower lip and shade jaw generally. The darkest areas will be neck, because also the face can cause extra shadow on there.
Also think of hair; the way light falls on it and creates some shiny, light parts; also that hair causes some shadows on face in the way as it bends.
4- This time the light source is on the bottom. The same logic as ‘top’, is that you draw equal amount of shadows on the sides of face and neck; middle of neck has the light the most.
Imagining a submerged ball into places makes the thinking process easier. How? Watch the chin, cheeks, noseball.
Also there are some points that you can use mid-tones, like right above eyebrows; those parts can be slightly darker than forehead’s tone, according to the peron’s skeleton’s curves.
Next week I hope to meet you for dramatic lighting. Happy sketching you all!
Link of skeleton drawing: