 When this happens, one retains the spirit of ETTR (maximizing signal-to-noise) by making the exposure as high as possible subject to the shooting conditions.ISO may then be increased to bring the image up to a desired brightness.But, since increasing ISO does not actually increase sensor exposure (instead the sensor's signal gain is increased), it should be applied only after the actual exposure (set by the "f"-ratio and shutter speed) has been made as large as possible subject to shooting constraints.