You can draw a realistic human eye using a 2H pencil and a 2B mechanical pencil. Adding detail such as iris highlights, a reflected light source and the eyeball's curve helps to make the eye look more realistic. Begin with a sketched outline, and work outwards from the pupil.
Lightly sketch the eye's outline using the mechanical pencil, then color in the pupil. Use layers of pencil to achieve the darkest possible pupil. Then, use a color blender to add a light circle of color around the pupil. This is the base layer for the iris.
Build up detail in the iris using quick, light lines to create shadows. Add darker shadows close to the pupil to give the eye more depth. Blend the shadows together while retaining some definition, then draw around the iris to define its outline. At this point, you can begin to add color to the iris.
Create light highlights in the iris using a kneaded eraser for precision. Once the iris has sufficient depth, add shadows around the iris before creating shadows between the eyebrow and eyelash. Continue to shade where necessary, and add details such as the eyelash reflection, tear duct and blood vessels.
Complete the drawing by adding the upper and lower eyelashes, adjusting their curve to suit the viewing angle.