An image sensor has a stacked pixel arrangement including both rolling and global shutter readout circuits wherein each pixel includes an adjustable transfer transistor gate voltage level for modifying electric charge within a photodiode during exposure depending on incident light intensity. The sensor also has a row decoder circuit providing readout signals to each row of the imaging cells during both a readout interval and during a calibration interval for each row. The row decoder may employ one of several of its features to provide a self-knee point calibration following an image signal readout in order to minimize photo conversion variations that lead to fixed pattern noise and to enhance dynamic range.