You can change the pivot point of the contrast buttons by sliding the zero position of the scale at the bottom of the tone curve. Drag the zero EV marker to the desired location, then hold the Option key (Alt key on Windows) while the contrast buttons are clicked and the curve will pivot at the zero position.
Manual Tone Curve Adjustments
Tone curves may also be adjusted manually. On the tone curve graph, there are a number of round dots. Each of these dots is a control that can be moved using the mouse. Begin by selecting the number of control points. Click on the Control Point selection button 
to choose between 5 or 7 control points. Each click will toggle between the 5 and 7 points options. The graph will instantly update to reflect the choice. To adjust a portion of the tone curve, click and drag the point on the curve to the new desired location. The prescan will update as the point is moved to show the result. The live update makes it easy to achieve the desired effect. As each point is moved, the line connect- ing the point to the neighboring points will rubber band to maintain a continuous transition in that section of the tone curve.
Holding down the Control key (Macintosh) or the Alt key (Windows) while
If a radical bump or sharp bend occurs in the curve, adjust the surrounding handles to maintain a smooth curve and minimize any chance of posterization.
The three buttons at the bottom of the tone graph control the range of manual adjustment allowed for the curve control points. The 
button restricts each control point to horizontal move- ments (+- EV changes) only. The 
button restricts each point to vertical (+- output value changes) only. The 
button allows full xy motion to the point. One restriction on the motion of these points is that they cannot be moved to left or right, above or below the same position as their neighboring points. This helps prevent many mathematical problems in creating the curves.
As each control point is moved, two lines are drawn on the tone graph to aid in positioning the point. One line is a vertical extending down to the EV scale at the bottom of the graph, and the other is a horizontal line extending across the graph to the output value tone scale on the graph’s left side. Numeric readouts appear near each scale to aid in precise positioning of the point. As each point is moved vertically, the output value will change as odd numbers when moving down and even numbers when moving up the graph. The increment will usually be by 2, except when rounding errors cause the number to increment by a single number. The numeric readouts, combined with the manual adjustment restriction buttons makes it easy to create curves very precisely.
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