Contrast (1/4 and 3/4 Tone)
Setting the Image's 1/4 and 3/4 Tones
Setting the 1/4 and 3/4 tones sets the image's contrast without changing the previously set white point, black point and mid tone to any extent. This is important because we do not want to undo the good we have done. This adjustment is made by visual evaluation of the monitor and works best with a color managed (calibrated and profiled) system. However, with a little practice and some test printing, you may be able to visually gauge outcome in an unmanaged system. The 1/4 and 3/4 tone settings are critical to setting the overall contrast of the image. The 1/4 and 3/4 tones settings are performed in the Luminosity mode to affect the previous color settings as little as possible.
Make sure the info Palette is visible on your screen. If it is not visible, select Window, Info.
Adjusting the Image's Mid tone
Go Layer, New Adjustment Layer, Curves. Make sure the Mode is set to Luminosity.
Make sure the value readout is set to Ink Percentage. Make sure the Preview box is checked.
Make sure you lock down the mid tone point by clicking on the line in the center of the graph. This will prevent the density from changing.