You control your duotone’s ink coverage by clicking on the curve box for either Ink 1 or Ink 2 in the Duotone Options dialog box. In the resulting Duotone Curve dialog box, you can adjust the curve to change how the ink is distributed across the image. The default duotone curve is a straight diagonal line, indicating that the tones are matched one to one; a 50% gray pixel will get 50% ink coverage. You probably don’t want both inks set to the same curve; otherwise, your image could be quite dark and heavy in the midtones. One option is to adjust the black ink curve to be lighter by moving all the points down on the curve a little
You can modify a duotone either by clicking and dragging a point on the curve or by entering percentages in the text boxes. Unlike regular curve adjustments, you adjust the duotone curv...