This training is going to go through the correcting of images in stages; taking it one step at a time. The first stage is to maximise the tonal range across the 256 tones available within the colour channels.
Reading the Image
Now that the image has been recorded and digitised, by either the camera or scanner, you will find that it is quite common for the tonal range to be less than the possable 256 tones that the colour channels can hold. To simplify things, it helps to first look at a greyscale image as it just has one 'colour' channel to deal with.
In this example there are strong tones from black to white. The highlights look a little too bright, but the shadows look more dark grey than black.
NOTE: If you do not see detail into the shadow areas in the image above, or if the shadows are looking solid black, then you are probably using a monitor that has not been calibrated. Monitor claibration is part of Colour Management which must be used if you are serious about colour correcting images. Even though the image above is a greyscale, the grey tones from black to white are still 'colours' from the colour correction point of view. Colour Management not only displays the reds, greens and blues correctly, but also the shadow and highlight values.
LINK: Read the page about Colour Management if you have not already done so.
Use the Histogram (found under 'Window' on the Photoshop menu bar) to give you the information required about the tonal range of the image. You may think that you can "see" how deep the shadows are, and how rich the highlights are, within the image above (FIGURE 1), but this is a subjective approach as each person will "see" these tones differently. The Histogram (FIGURE 2) tells you the blunt facts about the tonal range in a way that is common to all who view the image. Even if you do not have a colour managed monitor!
The Histogram displays the tonal range from "0" at the far left, to "255" at the far right. In this example the Histogram shows that there are no black tones at all as the shadows don't start until some way in from the left. The highlights are shown to have hit the end of the range at the right. In fact the small spike shows that there is a small section of the pixels have gone to the maximum white. Little can be done about this highlight now as the damage has already been done. However, since the Histogram shows that the highlights are only a thin line, meaning that there relativley few pixels involved, then this will be left as it is. It is the shadows that we will consintate on here as something can be done about them.
Correcting the Image Contrast
Open the Levels control panel (under Image > Adjustments > Levels on the Photoshop menu bar). This shows the same Histrogram as was seen in the Histogram panel viewed ealier, but now has the controls to do something with the tonal range.
There are three sliders under the Histogram in the "Input Levels" section. These are colored black grey and white to represent the shadows, midtones and highlights. Leave the highlights alone, as they have already hit the end. Move the lefthand slider (shadow) to the right so that it meets the shadow detail in the Histogram. What this will do is pull that point of the Histogram back to the left so that the shadows will start at "0" on the tonal range. Be careful not to take slider past the start of the Histogram detail as that will cause damage.
Now the image has a richer tonal range as the shadows have stronger black detail.
The Histogram panel now shows that the image has a full tonal range from "0" to "255". However, there are no extra tones here that have appeared by magic. The image had a narrow range of tones to start with. These tones are still here, but they have now been stretched over a wider range. Hence the "fracture" lines that now exist within the Histrogram.
NOTE: This a little like having a short loaf of sliced bread. You may want it to be longer, but you can only do that by moving the bread slices apart. You now have a longer loaf, but you gain no extra bread by doing so.
What Can Go Wrong
Like most things with images, caution should be used. If you rush things then you run the risk of damaging the image tonal range.
In this extreme example the shadow and highlight sliders have been pulled within the Histogram detail. All the pixels represented by the Histogram to the left of the shadow slider will now be condemed to solid black, and those pixels represented by the Histogram to the right of the highlight slider will now be condemed to pure white.
The result can be a major loss of tones, depending on how far you took it.
The resulting Histogram shows that the shadows and highlights have both hit the ends, and not by small amounts. This is known as "clipping" where much of the subtle shadow and highlight detail has been thrown away. Be warned, there is no retreating from damage such as this.
RULE #1: Make sure you are working on a copy of the original image. If you do go too far and damage the image, then you will always have the original to go back to and make another copy to work on. This is one of the major advantages of working with Camera Raw files as the originals can not be damaged, short of deleating them of course.
However, if you do want to throw away shadow and highlight detail, for graphic effect or artistic reasons, then of course you are free to do so. They are are your images after all. But normally the aim here is to reproduce the images with a full and rich tonal range; to present them at their best. Usually it will be the designer who has bought your images that will be "creative" with them. They may well crop the images, put text over them, and even put colour tints through them. But if you damage the images then you will reduce both the reproduction quality and the graphic choices that the designer may want to use.
BUT! If you really can't help yourself...
RULE #2: If you insist on damaging your images, then make it big, and call it "art". Be prepared to not sell many images though.