It is pretty easy to illuminate a person’s eyes properly if you use a flash. Although it is a really simple process, illuminating the eyes is a really important aspect particularly when it comes to portraiture. If you can position the light well, you are almost done!
If you are using a natural light, don’t let your subject to look directly at the sun. It is always better if you can find a relatively diffused light (either inside a building or under a shade). The best results can be achieved if you can find a surface that works as a reflector to direct light toward the face of the subject. Such reflector should be placed in front of the subject’s face.
If you use a flash in the hot shoe, however, you may have to reflect the flash output towards the side of the subject. In this case, you have to ask the subject to face towards you. You don’t need to bounce the flash off the ceiling; if you do so, your subject will have shadows under the eyes.
When it comes to a flash out of the hot shoe, you should place the flash in a large modifier. This modifier should be larger than the subject’s face. Then, you should place the light modifier (together with the flash), in front of the subject’s face. By asking the subject to turn the face slightly toward the light, you can expect better illumination.