3. Retinal imaging will be performed using a Canon Cr-DGi (Canon Co Ltd, Kyoto, Japan) nonmydriatic retinal visualization system. After accommodation to darkness, 1 image/eye will be obtained [Liu et al.]. Trained observers will identify individual arterioles and venules, using a validated computer-assisted program IVAN (Vasculo-matic Nicola, Ophthalmology and Visual Science, University of Wisconsin-Madison[Sherry et al.]). In addition, we will investigate visual acuity (clearness of vision, spatial resolution of the visual processing system) by the non-invasive adapted Snellen charts without visual aids.