Curves and colour casts

It takes practice to judge colour and colour casts in an image and adjust them with curves. If part of the image should be a neutral grey, that is a great help and we can use that as the guide.

• Open sample image number 04.jpg.

• Go to Window > Show Info, then go to Image > Adjustments > Curves.

• With the dialogue box open, the cursor changes to the eyedropper tool and the colour readings appear in the info box.
The small red circle is a suitable position to place the cursor.

• Put the cursor on to that spot, hold Shift and click. This locks the numbers into the Info box and they are identified by the number 1.

In this example, note how red is lower than green and blue is higher than green.

• At the top of the dialogue box, select Red in the Channel pop-up menu. We want to increase the red number in the Info palette until it is the same as the green value.

• Run the cursor along the curves line until the reading in the Input box is 116 or the value you have in your info palette. Click on the line to make an anchor point. If you cannot get the exact figure, click on the line at the nearest point.

• The Output number should then be changed to the Red value in the Info palette. Press the up arrow on the keyboard until the new red value equals the green value.

• Now select the Blue curve from the Channel pop-up menu. Run the cursor along the curve to find the position of the 152 value or the value you have on your Info palette. Click.
As before, the exact position of the anchor point should be adjusted by changing the Input box number.

• With the keyboard arrow, move the anchor point down until it equals the green value. The overall colour of the image will now be neutral, but the contrast is too low.

The red and blue values are adjusted until they equal the original green value.

• We don't have to OK the box yet; other adjustments can be done first.
In the Channels drop-down menu, select the RGB composite channel.

• Firstly, put an anchor point in the centre of the curve (position 1). This will prevent the curve from moving here.

• Then insert the other two point and move them as in the diagram. Point 2 peps up the highlights while point 3 enriches the shadows and blacks - together the improve the contrast.
Just how much shape is put in here is a matter of judgement.
When satisfied, click OK.

The image will now have clean,
neutral colour and stronger contrast.

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