Adobe Photoshop Tutorial. Copyright Michael B. Karbo.

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    27. The curves function

    When you master the Levels function, you can make most of the adjustments that you need. But for some experts adjustments with curves is the ultimate tool.

    If you are a newbie with Photoshop, then don’t expect the curves function to become your favorite right away; you can also manage easily with the Levels function. But read the chapter anyway and do the exercises – then you know that there are exciting possibilities ahead.

    Introduction to curves

    There are two types of ”curves” in Photoshop. The one (Bezier curves) deals with vector drawing. The other - the curves function, can make avdanced and extensive adjustments in the picture’s color tones. According to the aforementioned experts you can use curves for a number of tasks:

    ·     Adustment of the picture’s general light intensity and contrast.

    ·     Adjustment of light intensity and contrast in selected color tone areas.

    ·     Adjustment of the picture’s colors.

    Furthermore you can make much more precise adjustments with curves than with any other tool.

    Curves are thus an one-in-all very flexible solution. They can be used for small and subtle adjustments as well as drastic effects. Unfortunately adjustments with curves are somewhat more complicated to understand than are adjustments with levels, which are much more user friendly.

    Ready for the review

    You now need to read in a picture, so we can get started looking at the curves function.

    1.   Start by retrieving the picture eiffel2.jpg from the home page for this booklet.

    2.   Read the picture into Photoshop.

    3.   Then press Control+m, which is the shortcut to the curves function.

    4.   You might want to click on the small button in the lower right corner of the dialog box; then the box is enlarged:

    5.   Try to place the dialog box so it does not cover the picture. Now look closer at the Curves dialog box, without doing anything else:

    Figure 66. The dialog box Curves. The most advanced, flexible and most difficult adjustment function.

    Now you are ready to follow the review. The following section gives a more theoretical introduction to the Curves function. Then come some exercises

    Understand the Curves function

    The Curves funtion can change the brightness of certain color tones. It has two scales, one for input values and one for output values. In the Levels function you also work with in- and output, but in Curves it is more important to keep an eye on the two scales, since there is no histogram to support you.

    The scales (or the axis in the co-ordinate system) show tone values from black over middle tones to white. The input is seen on the horizontal scale. That stands for the picture’s current color tones. The vertical scale shows the output values, which are the color tones after the adjustment:

    Figure 67. The curve is placed between two scales, which indicate the color tone values before and after the adjustment.

    When you have not made any adjustments, the curve is seen as a 45-degree straight line as shown in Figure 67. It shows that the input values are identical to the output values. So there was no change in the color tones.

    Examples of general adjustments

    When we change the curve – drag it away from its center position – we see a change of the color tones. The output values get different than the input values.

    See the curve in Figure 68, where the whole middle part of the curve is dragged down and to the right. This means that the output values are dragged down toward the dark end of the vertical scale, can you see that? So a given middle tone will get somewhat darker because of this adjustment:

    Figure 68. All middle tones are changed from the light toward the dark. The picture gets generally darker.

    If the curve is dragged in the oposite direction, the result will be opposite; the picture gets generally lighter in the middle tone area:

    Figure 69. This adjustment gives a much lighter picture.

    The adjustments in Figure 68 and Figure 69 are very drastic. You would very rarely manipulate the curve so far away from the middle position; the two illustrations are just to show the principle.

    Ordinarily you would use much smaller adjustments. See the so-called S-curve in Figure 70. There the curve’s middle point is unchanged (by the mouse cursor). The light tone area (by point B) is dragged upward and to the left. This means that the light tones get a little lighter. The dark area (around point C) is dragged downward and to the right, so the darkest half of the tone area gets a little darker.

    The result is that the line section between points B and C gets steeper than the 45 degrees, which was the starting point. And when a curve section is steeper than 45 degrees, it means that the constrast is increased in the corresponding tone area. That matches our example also: We made the light tones lighter and the dark tones darker, and that is the same as increasing the contrast.

    Figure 70. The so-called S-curve increases the contrast in the middle tone area. Notice the grid lines.

    You manipulate the curve by inserting anchor points like B and C in Figure 70. You just click on the curve, which inserts a new point. The two end points A and D always define the extent of the curve, but you have to insert the other anchor points.

    There can be up to 16 anchor points on a curve, so there are wide possibilities to adjust individual sections independently. But often it is sufficient to insert just one ot two anchor points.

    Try for yourself

    You have the picture eiffel2.jpg on the screen. You see some buildings, which are photographed from above. The problem is that the picture was taken on a very misty day, and therefore it has no contrast whatsoever. Neither are there any real dark or light details.

    1.   Press Escape to close the Curves dialog box, if it is still open.

    2.   Choose a 100% showing (you could use Control++). Use the hand tool (with the space bar plus dragging with the mouse) to make the upper part of the picture visible.

    3.   Press Control+m again to open the Curves dialog box. When you have to work with curves in your own pictures, it is best to use adjustment layers, as you have seen previously. But we will skip that for now.

    4.   Use the mouse to place the dialog box in one side of the window (it should not be in the middle of the picture).

    5.   You need to liven the picture in its dark and light areas. Start with the light; drag the top end anchor point (A in the figure below) horizontally to the left. Follow the effect in the picture while you drag.

    6.   You can also use the arrow key ArrowLeft once you get hold of the anchor point. Drag the point to a position as shown below (with input = 226). You could also enter the numerical value in the Input field:

    7.   Then grab the opposite end point (D), and drag that horizontally to the right. Again it is easiest to use the arrow key ArrowRight once the point is selected. Here you need to get rather far to the right before the dark tones ”take hold”. Try to drag to an input value of 61:


    8.   That helps a lot in the picture’s color tone balance.

    9.   Finally you can try a new way to place a point somewhere between A og D: move the mouse cursor into the picture itself. The cursor canges to an eye dropper.Try to hold the mouse button down while you drag the eye dropper around in the picture. See the point on the curve, which moves up and down. The point corresponds to the color tone, which is in the eye dropper’s area.

    10.   See if you can find the green wall, which is seen near the top center of the picture. Hold the Control key down, and click with the eye dropper in the green area. That places a point on the curve:

    11.   Now use the ArrowUp and ArrowDown arrow keys to adjust the output value. Follow the effect in the picture. For each ArrowUp press the tones get a bit lighter. Each ArrowDown press darkens the tones. Usually you only have to adjust just a few steps up or down to get the desired effect.

    12.   Try for yourself to insert additional points on the curve, and see if you can improve the color tone balance. Here we made an attempt:

    The curve needs to be smooth and contiguous. Then the color tone distribution looks natural.

    13.   You need to stay with very small shifts of the individual points. But you can always delete a selected point by pressing the Delete key.

    14.   You could save the pictue in a new version. Close it.

    It is quite easy to use the Curves function in pictures where there are major flaws in the color tone distribution. That is usually done with just a few points, as you just saw in the exercise. You started by adjusting the two points A and D, and that had a major effect. It is somewhat more difficult to make the small curve adjustments. It requires a good screen and good eyes to optimize the colors in an otherwise well balanced photo. But you can experiment with that.

    28. Adjustment of colors

    Now you are going to try adjusting some real ”poor” pictures. They are mis colored pictures – the colors are quite unequally distributed. You can correct that with the Curves function.

    The pictures can also be repaired with the functions Color Balance, Variations and Levels; many will probably find that somewhat easier. But here you will take the direct and hard way: Curves.

    A rescue action

    We have scanned some slides from around 1970. To start with the pictures were underexposed – and time has taken its toll on them.

    When you scan slides, you can usually adjust with curves before they are scanned. That gives a better picture material than when you adjust after the scanning, since any adjustment results in loss of picture details. In order to show the color adjustment in Photoshop, we have scanned the slides without any form of prior adjustments. So there is something to work with now!

    Now retrieve the picture dias1.jpg from the home page for this booklet, and read it into Photoshop. The first picture is from Mallorca and shows an idyllic old windmill. But – as it sometimes happens with old slides – the colors became badly skewed toward the red. What if this was a precious photo – one you really want to save for posterity? You treat it with the Curves function!

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