Brother MFC-8420 User Manual page 261

To change the default settings of the scan, click on the Scan
Settings tab.
You can set the Color Mode to Black & White, 8-bit Gray or 24-bit
Color.
The resolution can be changed from 100 to 1200 dpi. You can define
the size of the paper you're using, and you can adjust the brightness
and contrast of the image before scanning.
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