No two manga artists will draw eyes the same way (unless they were copying each other, of course). One can usually identify a familiar series or a popular artist via the eyes of a character alone. (Ex: Naruto, Bleach, Clamp, Arina Tanemura) In fact, not only will the eyes differ between different series, but they will differ within one as well. For instance, male eyes will differ from female eyes in that they are usually smaller in height. This gives them a more mature and masculin look. Innocent characters usually have wider, larger eyes than evil villains. A character's mood and even personality will be often reflected through the eyes; thus, it is important to know more than one way of drawing them.
Let's first look at different eye structures. You can basically draw the shape however you want and the easiest way to find the perfect set of eyes for a specific character is to experiment. Notice how a small change in the shape will completely change the look of your person. The following are just some examples:
Eyes between genders will differ. As mentioned before, masculinity is usually associated with smaller eyes while innocence and cuteness with larger ones.
Eyes between ages will also differ. Children are usually drawn with larger eyes than adults. As they grows, their eyes will change depending on their life experiences. Thus, if you want to draw an adult who lived a carefree life and simply never grew up, you draw him with large eyes to reflect his inner childish personality. If you want to draw a girl who was forced to experience the hardships of the world at a young age, you would draw her with eyes usually only given to adults, etc.
In addition to reflecting a person's personality, eyes are also able to fully convey emotions that even words can't describe.
In the (very short) comic above, the character's smile seems sincere enough. In the second frame, he observes whoever is walking away with a happy look.
The character's smile in this version seems fake. Clearly, he is experiencing some pain that he doesn't want the person walking away to see.
And this one is just plain evil. He's definitely up to no good. Definitely.
So as you can see, simply changing the eyes in the second frame will change your perception of his smile as well as give you insight on what exactly the character is thinking in his head.
