To do this right, it really helps to have a macro lens. You can try to do this without, but it is difficult to get close enough for the eye to fill the entire frame. I would recommend 100mm or more for your macro lens. Shorter focal lengths mean you must be closer to the subject and possibly blocking the light or reflecting in the glossy surface of the eye. You can still get a good photo with a shorter lens—don’t let equipment keep you from trying! I’ve taken very cool eye photos with a macro attachment for an iPhone.