Canon 970 IS - Manual (Page 146)

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z When there is not enough space left on the memory card,
red-eye correction cannot be performed.
z Although you can apply red-eye correction any number of
times to an image, the image quality will gradually deteriorate
with each application.
z Since the correction frame will not appear automatically on
images that have already been corrected once with the Red-
Eye Correction function, use the [Add Frame] option to
correct them.