The realistic eye drawing you see below resulted from making a basic line drawing, adding tones to the drawing, and then blending those tones together.

<a href="http://www.letsdrawpeople.com/wp-content/uploads/2015/04/how-to-draw-an-eye-0307-lower-eyelid.jpg"><img class="aligncenter size-full wp-image-1874" src="http://www.letsdrawpeople.com/wp-content/uploads/2015/04/how-to-draw-an-eye-0307-lower-eyelid.jpg" alt="how to draw an eye 0307 lower eyelid" width="800" height="618" /></a>