Preview mode low-resolution readouts occur, and then a shutter button on a camera is pressed, which causes an image sensor cleanout operation to occur. Following the cleanout, a high-resolution readout occurs. As rows of sensor values are read, the first rows are rows corresponding to a pre-defined horizontally-extending shielded area. There are no valid area sensor elements to either side of the horizontally-extending area. Data values read from the horizontally-extending area are used to determine optical black (OB) values that are then used to adjust the valid area values read out of the image sensor in that same frame. The same OB values are used throughout the adjusting of the valid area values of the entire frame. No values from the preview readouts are used in the OB value determination, so there is a clean break between the preview mode OB level and the high-resolution capture OB level.