In-camera Peripheral Illumination Correction

Several EOS models feature in-camera peripheral illumination correction to help even out brightness across the frame, especially in the corners. If you find that even with this enabled the corners are still darker than you expect, it may be due to the ISO speed you are using. At higher ISO speeds the amount of correction applied to the image will be reduced. If you wish to achieve a greater level of correction, either reduce the ISO speed and re-shoot, or use Canon’s free DPP software where a greater range of adjustments are available.
Camera settings