(Long Zephos post alert! As if I actually do short posts, hahahaha ^_^)
Ah, the eyes ^_^ Not enough can be said about the good old windows to the soul in anime/manga style... errr... especially in regards to the healthy female bipeds.
Eyes in anime/manga work is pretty much an artists signature and it's pretty normal for even a professional artists to constantly refine them to optimize appeal and execution.
Anyway, outside of tutorials and practice, the number one recommendation I have to give for new anime/manga style artists when drawing eyes is this:
1. Find out what your favorite anime/manga eyes are, then compile and analyze them. Really study them for what they really are and see them in shapes and in perportions.
2. Composite individual features of each eye to form an achetype of the "perfect eye" that you want to emulate, such as the thickness of the upper lash, the size and location of the highlights, the size of the pupil, the spread of the lashes, the tallness of the eye versus the length, the lower eye line, eyelid and eye brow distance ratios, the distance between the eyes, and the default openess of the eye. With the last one, some artists naturally draw the eye fully open by default like Arina Tanamura and Koge Donbo, others aim for a half open sexy default look, like the "bloodline" of Studio Pierrot).
3. Try to refine and establish it in your work in mutliple angles and situations to see if it is practical and effective in actual drawing.
If you are inexperienced, you might not be able to downsample and break down advanced eyes and think of them both in color and in black and white when working, but that's why artists need to practice in the first place.
Of course, the simplest and easiest eye to draw is a black dot, but like Tri_kento said, try and have the detail of your character's eyes balance with the rest of the drawing. If not balance, they should take priority over the rest of the drawing unless the mood calls for obscurity. Drawing recognizable eyes is easy, drawing eyes that balance between distinct, beautiful, efficient and versatile in execution that screams off the page with heart felt soul is a different game. Of course, this is all taking into account, along with more importantly, if it fits the character and genre, as there are many professional artists who use very simple eyes, simply because complicated eyes would throw off the balance.
Some artists have the ability to trick the viewer into seeing eyes as being more complicated than they actually are. Koge Donbo is a master at this with her trademark "Egg in a Nest" eye style, and the funny thing is that in some of her work, the insides of the eyes are filled with 95% black with suprisingly small highlights (Novices might think highlights = cute, but the opposite can be equality true), the only sign of color is a small line at the base of the eye. As an artist who has found a large corner in the cute market, this comes as a surprise to most of her fans who study her style.
As I mentioned earlier though about the Studio Pierrot style eyes (I used the term bloodline earlier, cause it seems that most every artist who has walked away from there borrows a similar technique for eyes).... If you've ever seen an artist from Studio Pierrot, they draw some of the absolutely most beautiful eyes out there and generally have a glossy effect with minimal use of pure black and a clear showing of the pupil and iris with wild lashes... it could be said that they serve up the caviar of anime eyes, which is to be expected of the studio that pretty much invented the magical girl genre... pretty funny for a company that's named literally after a clown.
Those two above are my personal recommendation for elaborate female eyes, but for a more down to Earth, animatable eyes, Rumiko Takahashi and Akira Toriyama are perfect, as they've been around since the early days of anime and have the base technique that many have since built on (Unfortunately, the problem with Toriyama is that his eyes are TOO distinctive, hahaha). Ken Akamatsu is also a perticularly good choice for the "great looking but simple to draw" variable. There's no better way to learn than to study the masters after all and practice yourself into a coma.
Anyway, there's a lot of unwritten rules to be said about drawing eyes. I also personally find them to be one of the most rewarding parts of drawing in anime/manga style. The tutorials will carry you pretty far, but the rest is up to personal discovery. Of course, these are more of ideas than instructions, but I hope you would find them useful.
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Zephos's ice creamy anime and manga art, featuring Xynthica, a fluffy little female biped who works as a summoned monster for endorsing products -- instead of killing things! Contains cute colorful cartoons and caustic cultural commentary!