 The ETTR exposure is, by its very nature, established with a camera ISO setting that allows the exposure indicators (right edge of histogram or blinkies/zebras) to indicate when the sensor is at or near saturation for desired highlights.Most people will find this to be the camera's base (lowest, not false) ISO.However, depth of field requirements may demand such a high "f"-ratio, and motion-blur/camera-shake issues may demand such a fast shutter speed, that ETTR is not possible at this ISO setting.