 A new fundus-guided microperimeter, the MP-3S, has been developed by Nidek, Inc. to track the fundus of the patient and present stimuli in specific anatomically-defined locations. Furthermore, this tracking means that exactly the same locations can be tested on subsequent follow-up visits. The investigators will use a method called two-color perimetry to map rod and cone sensitivity on this device. With this technique, the sensitivity difference blue-red to chromatic test stimuli can be used to determine whether rods, cones or both photoreceptor systems mediate the threshold at a given location in the macula. 