Canon iR3245 Manual page 402

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If you make fine adjustments to the density, the automatic scan exposure setting is cancelled, and the
original is scanned according to the Fine Adjustment settings.
4-148 Prevent Bleeding
This setting prevents dark areas on the
reverse side of the original from
appearing on the recorded pages, but
light areas on the front side may not be
scanned either.
This setting may not be able to
completely prevent dark areas on the
reverse side of the original from
appearing on the recorded pages, but
light areas on the front side of the
original are always scanned.