Drawing A Realistic Eye

Eyes are one of the most commonly drawn subjects for an artist. Getting them right means everything, and you don't need to draw an entire face just to practice it. This step-by-step tutorial will give you an example of how one artist approaches it, noting that there are many approaches to drawing the human eye.  

"Start with the eye in your sketch. The iris is where you really add the most detail. They should feel watery. Eyelids should curve as they dictate what the nose, cheeks, and lips will look like." – Artist Morgan Davidson 

Begin by shooting an eye that you feel would make a great subject. Your phone’s built-in camera will work as long as the lighting is adequate enough to illuminate the color of the iris.  
Rather than color the pupil black, you can achieve that color another way, using Indigo Blue (PC 901) to provide a base color for the pupil, choosing highlights, as the pupil is naturally reflective.  
Layer in Dark Umber (PC 947), blending with the Indigo Blue to create black. Next, use True Blue (PC 903) for the iris.

A Putty Beige (PC 1083) is layered in to add in more neutral tones. As each layer is added, the colors become more waxy and easily move on the surface. Darker edges exist mainly on both the outer edges of the iris and closer to the pupil. Try to make these lines directional to give the illusion of color radiating from the pupil. Burnish the colors with White (PC 938) and add a little depth on the outer edge of the iris. Dark Umber near the top of the eye gives the effect of a shadow from the eyelashes. It's important to desaturate a bright color with neutral tones. Remember to periodically use a colorless blender. Darker and lighter patterns within the iris create more interesting and realistic patterns. Continue pushing the contrast further. Highlights are added to the pupil to give the illusion of realistic watery eyes. 

With the White, establish the "white areas" of the eye. Follow that layer with a layer of Cream (PC 914), then White again, creating a buttery surface where additional layers will be easily blended. The white portions of the eye are rarely white, rather they're a mix of colors. Go back and forth, with darker and lighter values, building depth and adding shadows along with colors that add realism.  
An application of Light Peach (PC 927) around the eye blended with Dark Umber and Cream further defines the rest of the eye. Now it's time to add color to the eyelid, using the White and Light Peach.  
Continue layering the Light Peach with Dark Umber (PC 947) for the skin around the eye, including the eyelid and up into the eyebrow area.  
Add in eyebrows and then eyelashes until the eye is complete.

The eyes are the gateway to the rest of the face, and will be what most people will immediately focus on. Take the time to draw them again and again. They will eventually become second nature.