Adobe Photoshop Tutorial. Copyright Michael B. Karbo.

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    Adjustment of color channels

    In the following we will show

    how you can use Curves to adjust both brightness, color balance and contrast. We do not give a final recipe for how the adjustment has to be done, but we show a work method. The idea is that you have to experiment.

    1.   You have the picture dias1.jpg on the screen. Save it in the Photoshop format as dias1.psd. Then read these instructions, where you have to insert four adjustment layers:

    2.   Select menu items Layer --> New Adjustment Layer --> Curves four times.

    3.   Name the four new layers RGB, red, green and blue. When the dialog box Curves appears, just click on OK; we will adjust later. When the four new layers are inserted, The Layer palette will look like this:

    4.   The RGB layer needs to be selected. Then double click on the curve icon here:

    5.   That opens the dialog box Curves. The first adjustment shall give light in the picture. Drag the top right end point well to the left. Then place one or two new anchor points on the curve like this:

    6.   See the effect in the picture. Was it improved? Then click on OK.

    7.   Then select the layer red. Now we will adjust the red colors. Double click on the curve icon, to open the dialog box.

    8.   Select the red color channel in the Channel field:


    9.   Now you are working only with the red colors. They are way too dominating. So make an adjustment like this:

    10. Click on OK when you are satisfied. Then work in a similar manner on the green and blue channels: these colors need to be accentuated relative to the red channel, so the curve needs to be quite different. Start with the layer green, select the green channel (you can do that with the shortcut Control+2) , and make an adjustment (see Figure 71).

    11. Finally adjust the blue channel in the layer blue. Experiment or make adjustments similar those in Figure 71.

    12. Save the picture when you are done.


    Figure 71. Adjustment of the green and the blue channel.

    Comment on the exercise

    You have probably corrected the colors in the old picture from Mallorca. The picture is not particularly interesting in itself (it is also scratched). But now you have tried the technique by using curve adjustments in the individual color channels. You can actually do the same with Levels function.

    You now have four adjustment layers in the image file dias1.psd, each with a curve adjustment. The advantage of the adjustment layers is obvious:

    1.   Try to open the adjustment in the layer red (by double clicking on the curve icon in the layer beam).

    2.   Press Control+1 to see the red channel.

    3.   Voil --> , then you have the curve with your adjustment. Now you can change it a bit:

    4.   You can immediately see the result in the final picture, where all four adjustment layers are active.

    5.   Open also the layer green. Ativate the green channel with Control+2. You could try if you can improves this adjustment.

    6.   Close and save the picture when you are finished.

    By adjusting with curves in each of the three channels (red, green and blue) we get access to regulate the color balance. We can thus correct a red dicoloration, like you just tried.

    The disadvantage in this technique is that you in principle have an equation with four unknown factors, and there is no solution to that. You can adjust each of the four layers independent of each other, but the combined effect can be hard to control.



    Figure 72. With some work an old and ruined photo can easily be restored.

    Exercises for you

    Adjustment with curves is difficult – it requires some training. But the method is flexible and effective. Curves are by all means the experts’ favorite tool.

    We will now ask you to do four exercises with four pictures. Here are the instructions:

    1.   Get the pictures dias2.jpg, dias3.jpg, dias4.jpg and dias5.jpg from the home page for this booklet.

    2.   Save each picture in the Photoshop format (as PSD file).

    3.   Now work on each of the four pictures in this way: insert an adjustment layer for curves. Make a general RGB adjustment, and see how good each picture can get. Take your time for this!

    4.   Look back in the booklet and see the curve adjustments we used for the picture dias1. There you can get inspiration. It is important that you don’t just drag casually in the curves; it might give smart effects, but it does not solve the problems with the color tones.

    5.   If necessary, you can insert more adjustment layers and use curves for the individual color channels (red, green and blue).

    If you can’t figure it out with the Curves function, you can use the Levels function. Use the same procedure with adjustment layers and use levels for the individual color channels.

    If you want an even easier use interface, the try the function Image --> Adjustments --> Variations. That is a dialog box where you can increase and decrease light intensity and the different colors by clicking on a number of sample pictures:

    Figure 73. The dialog box variations is used to adjust colors.

    Adjustments cost details

    When you work with adjustments of photos, remember that any adjustment costs picture details. The original image file will always have the most details. That holds true regardless of how dark or otherwise imbalanced the picture is. So always save the original version (also called ”the digital negative”).

    Adjustments can make the picture ”prettier” and more ”readable”. But the reworking also costs details – A little bit is lost in each operation. The advantage in Photoshop’s adjustment layers is that you can always change the adjustment, and yet not lose any picture details. If you don’t use adjustment layers, you will simply ”destroy” the picture more and more for each adjustment you make.

    If you scan pictures (or slides and film negatives), it is best to make the adjustment before scanning. Then you get the maximum number of picture details in the final image file.

    Creative curves

    You often read that you can use the Curves function to make very creative and smart effects; that is also true. Try this:

    1.   Get the picture market.jpg on the home page for this booklet and read it into Photoshop.

    2.   Insert a curve adjustment layer.

    3.   The curve needs to be opposite from the normal. Start by lifting the bottom left anchor point all the way to the top, and then move the other anchor point down:

    4.   Then click OK. Think about what the above curve involves. The color tones are reversed, right? Light becomes dark etc. – the picture becomes a negative!

    5.   Open the same adjustment again (by double clicking on the curve icon in the layer beam). Now change the curve to this:


    6.   That gives a so-called solarizing effect, where only some of the color tones turn negative, can you see that?

    7.   Insert yet another adjustment layer to Curves. Make the opposite curve than the previous (thus a U curve). Now the solarizing effect gets even more pronounced.

    8.   Save picture as market.psd.

    Figure 74. The curve adjustment resulted in a partially negative effect.

    Choppy curves

    Normally you would use soft and contiguous curves. That gives color tone transitions, which are natural in color photos. But you can easily break the curve up in separate line sections, and that can give violent color effects.

    1.   Remove the showing of the two adjustment layers (remove the eyes in the layer beam).

    2.   Insert a new curve adjustment layer in the image file market.psd (in top of the layer palette).

    3.   Click on the pencil button in the bottom right below the curve. Now try to draw a non contiguous curve like this, while you follow the effect on the picture’s colors:


    4.   You can see that it gives some new possibilites.

    The Curves function can create crazy color effects in any picture. The problem is that the effects can be difficult to control.


    Let us conclude the review with four minor tips for the work with curves and levels.

    Adjust individual areas. We have through all the exercises worked with whole pictures. But you may also find a need to adjust selected areas in a picture. That is done by making a selection and then do the adjustment in the selected area.

    If you first insert an adjustment layer, the selected area is automatically converted to a layer mask (as seen in the Layers palette). That is really smart, since it gives you the option to work more on the mask with brushes, filters and other effects quite independently of the other layers.

    Figure 75. The adjustmnt layer is created from a selected area. It gives a mask, which can be worked on further.

    Reuse a curve. If you open the Curves function with Control+Alt+m, you will see the same curve adjustment, which you used last.

    Grid disintegration in curves. Hold the Alt key down and click in the curve area to get a finer grid.

    See back/white areas in levels. The Levels function has a smart finesse, which many do not know. It may be hard to explain, so try for yourself. When you drag in the triangle (see Figure 62 on page 127) to the far right under the histogram, you convert the lightest color values to pure white. In the other end of the scale, the darkest values become pure black – depending on how much you drag the little slider to the left.

    If you hold the Alt key down while you drag in the sliders, you get help. Photoshop will show in the picture itself which pixels will be affected by the adjustment. So you get a visual picture of which color values will be converted to pure white (or black respectively). That is a smart

    Detail, which needs to be tested in practice,

    Figure 76. The Levels adjustment shows which color values will be foced to become pure white.

    Now you are at the end of the road. There are no more exercises in this booklet. We hope that you have enjoyed our review, and that you got encouraged to cavort in all the exciting Photoshop functions.

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