For eye detection, it is important to crop the input image to just show the approximate eye region, just like doing face detection and then cropping to just a small rectangle where the left eye should be (if you are using the left eye detector), and the same for the right rectangle for the right eye detector.
If you just do eye detection on a whole face or whole photo, then it will be much slower and less reliable. Different eye detectors are better suited to different regions of the face; for example, the haarcascade_eye.xml detector works best if it only searches in a very tight region around the actual eye, whereas the haarcascade_mcs_lefteye.xml and haarcascade_lefteye_2splits.xml detect work best when there is a large ...
