Drawing eyes is not necessarily hard, but very important. In portraiture, eyes are the part of the face which focuses attention of the viewer the most. Today I'd like to review anatomy of the eye in a way that is understandable for a painter, point out things that need extra attention and hopefully motivate you all to sketch a little bit ;)
Of course if you have any questions or comments, I'd be very happy to hear them.
We obviously all know that eyeball has a shape of a ball. The problem is that we rarely realize that while drawing - more often we just have something round and dark (iris with pupil) on something white. That is because we see even less than a quarter of the whole eyeball and do not think about the rest of it. So let's revise how the eye is built, for the starters.
Let's imagine eye as a ball, covered in thick rubber suit. Now, coming from one side to another, we cut a slit in that suit, so rubber covering moves apart and from the inside you can see a white ball. This is how we should imagine eyelids on the eyeball. They cover it, stick to it and move based on how eye moves, but we will come back to that later. For now, imagine eye as a ball in rubber suit ;) On this ball (let's take it out of the suit for a moment) we have iris, which is a lot like slightly concave bowl (like a hole) and cornea, which is also like a bowl, but convex one (upside down, like a hill on the eye). Cornea is transparent, so you can see iris behind it. Pupil is placed on the iris, in fact, it's more like a hole in the iris which let's light in.
When drawing eyes together, you can think of them like they are placed on the same stick and roll up and down together. When you look right or left both eyeballs rotate about the same angle. It's eyelids that make eyes look different.
Eyelids are pretty tricky. They have their thickness which is clearly visible, we should normally show the bottom plane of the top lid and the top plane of the bottom lid. When it comes to shading, light source is usually above the eye so the bottom plane of the upper lid is in shadow and the corresponding surface on the bottom lid is highlighted.
Shape of the eyelids is somewhat tricky. Please note, that most of time it's not symmetrical! We also have many many many different shapes, you can see some examples on the picture below. Male and female eyes are pretty much the same, as there is no significant difference. Female eyes appear to be bigger because female face is generally smaller. Everything else is just make-up ;) Men usually have even longer eyelashes! When it comes to children, their iris is the same as for a grown up (eyes do not grow with age!) but eyelids are smaller so iris seems to be big. Old person is more likely to have baggy eyelids.
To make things more complex, eyelids change their shape when we move the eyes. Cornea has a convex form and it will push the lid out. Take your time looking at friends, try to realize how cornea affects shape of the lids.
While refining details be aware that for example tear duct can come in a variety of shapes - round, small, big, sharp, whatever you like, in fact. Eyelashes should be grouped into chunks and a little curved. They are longer in the middle part of the eyelid and a bit shorter near the edges. One final thing to note is shadowing the iris. As the iris is like a crater and cornea is a hill, they will be shadowed in an opposite way (concave and convex shapes). But cornea is transparent, so only highlight will be visible. So for a simple summary: shade iris near the top lid and leave some light to the bottom part of it. Then add highlight on the darker part (that highlight comes from cornea, which is lighter at the upper part and darker at the lower part). That recipe works for light coming from above. For the light source below the eye just reverse the pattern. Also don't forget to shade the white part of an eye - eye is round and has a natural shadow. One last part is shadow coming from the upper lip and from eyelashes.
Just for the record - some simple sketches I did, just to practice positioning eyelids and such. Those were done very quickly and you should try doing this too! Just open Google, type in "portrait" or "face" and sketch eyes, as many as you can. Try to remember all the important things which I pointed out and check if you didn't forget anything. Is the shape correct? Eyelids look natural or not? Are both eyes looking at the same point? Are eyelashes moving correctly along with the lids?
Now you are ready to jump into some serious painting! :D Good luck!