An improved on chip test method for determining the photon transfer curve (PTC) and dark current in an image sensor is described. Cost and time savings is achieved by reducing the number of frames necessary for the measurements to three including two exposure frames and one frame for dark current testing. A conventional test involving “n” different exposure times each with two frames is replaced by implementing a snap shot mode where a first plurality of pixel rows are exposed for a time t1, a second plurality of pixel rows are exposed for a time t2, and so forth up to an nth plurality of pixel rows exposed for a time tn where the total number of pixel rows equals a frame and tn>t2>t1. The resulting image has “n” regions each with a different brightness that become progressively brighter from top to bottom of the image.