Canon MultiPASS C80 Software User's Manual page 180

If you use threshold as your scan option, since it is by definition a high
contrast image mode, you can only adjust the brightness of the preview
image.
To adjust the brightness or contrast for a black and white image:
On the C80/C70 tab, under Black and White Options, use the available
slider to adjust the brightness or contrast.
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These tone adjustments are only for black and white images. If you select
greyscale or colour as your scan mode, you adjust the contrast and
brightness of the image only on the Tone tab.
6-28 Scanning Chapter 6