The eye light is a piece of the lighting puzzle that is often overlooked in non-professional productions. As unimportant as it may seem, that little sparkle of the eye will add a significant amount of emotional depth and a sense of life to your on-screen talent, allowing your audience to better connect with the character. Since much of the talent's emotions are expressed through their eyes, it's important that the eyes are well-lit and able to reflect the emotion the character is portraying.
To maximize the effectiveness of this light, dedicate a specific light as the "eye light." The eye light should be a small, low-powered light placed near the camera. Though the eye light doesn't need to be exactly in line with the key light, it should come from the same direction. For example, if your key light is high, so should your eye light. This will help in making that sparkle appear to be coming from the appropriate light source. Remember, the eye light is supplemental to your light setup and should be adjusted to aid your talent's focus not bother your talent.
Make production easy on yourself and use the eye light as a guide for focusing on a subject. You're really looking for the reflection found within a person's eye and that reflection of the light will be come most defined when the focus is precise on his or her pupil. Since most people have dark irises, this intriguing and colorful section of the eye is also game for your eye light.
Whether you are using a high-key or low-key lighting scheme, throwing an eye light into the mix will help your characters come to life. Adding that sparkle to their eye can greatly enhance any emotion, making each scene much more compelling. To learn more about eye lights and where to place them, check out Lighting: The Eye Light