Canon MultiPASS L6000 Software Manual page 144

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 L6000 tab, under Black and White Options, use the available
slider to adjust the brightness or contrast.
n
These tone adjustments are only for black and white images. If you select
grayscale as your scan mode, you adjust the contrast and brightness of
the image only on the Tone tab.
6-26 Scanning Chapter 6