2.     Colour Balance

The "Colour Balance" tool seems to be favoured by many Photoshop novices. It's relatively intuitive and quite flexible in the adjustment choices it gives. We can work on "midtones", "shadows" and "highlights" independently, but there are hidden dangers in this approach. Realistically, if we decide to use this tool to make colour corrections it's best to stick only with "midtone" adjustments.

 

 

The screenshot shows that I have increased red in the "midtones" in an attempt to reduce the cyan cast to the image. The difference between the Before and After images is most obvious in the "Church driveway".

 

Before	After

By toggling the "Preview" On and Off will we can see a before and after effect. As mentioned above the colour balance tool is reasonably intuitive and so relatively easy to use. However, it's also pretty limited in what it can achieve, so I don't think much more time needs to be spent explaining how to use it.

3.     Levels Tool

The "Levels" option is probably the tool most Intermediate Photoshop users choose and is certainly a lot more useful than either of the tools described previously. As can be seen on the dialog box below it offers a histogram, as well as two sets of sliders, and three "eyedropper" tools. With the "Levels" tool we can also adjust image brightness, contrast and colour, but in a much more controlled manner than the brightness/contrast or colour balance tools.

 

Photoshop 7 Levels

 

Basic tone correction using levels

Looking first at the RGB histogram, the horizontal axis indicates the distribution of dark to light pixels in our image, and the vertical axis the number of pixels at any given lightness level. From this we can determine that for the above histogram there are no totally black (left end) pixels and only a small number of absolute white (right end) pixels in the image.

If we begin by considering the "Input" slider bar we find three triangles, black, grey and white. These triangles represent the "shadow", "midtone" (brightness/contrast or gamma) and "highlight" adjustments. Moving the "shadow" and "highlight" slider towards the centre will increase image contrast. Moving the "midtone" slider to the left brightens the image, whilst moving it to the right darkens the image. Ideally we would move the two outer triangles to just clip the edge of the first group of pixels (see screenshot below). However, it isn't always the case that the point at which the data begins and ends in the levels display is giving us a true indication of the actual spread of image data.

Photoshop 6/7 and CS has a feature called the "Clipping Display" that enables us see the effects of an "Input" slider adjustment. Simply hold down the "ALT/OPTION" key while moving either the "shadow" or "highlight" slider towards the centre will display exactly which pixels are being clipped (see screenshots below).

 

Adjusting the Highlights for "Clipping" display

 

Highlight Clipping display

 

The above screenshot exaggerates the technique slightly so that the reader can better appreciate the impact of moving the "Highlight" slider too far. Typically we would want to stop when the image only just begins to appear. In the above example we can see that the blue channel has been taken to saturation (level 0) and a small section of cloud has been "bleached out" to white. A similar exercise for the "Shadow" clipping display produces the following results.

 

Shadow Clipping display

 

The levels  "Output" slider bar allows us to reduce image contrast by compressing the data and making a dark pixel less dark or bright pixel less bright. The "Output" control can be used for targeting an image for particular print characteristics, e.g. setting the maximum black at level 5 and minimum highlight at level 250. This feature can be quite helpful when the user finds that their particular printer model tends to "block" what should be perfectly printable "shadows" or "blows" well detailed "highlights" out to white .

Basic colour channel corrections using Levels

So using the RGB "Levels" dialog we have managed to adjust the tone of our image. However, one of the main advantages of levels over "Brightness/Contrast" is colour correction. What about colour correction?

By selecting the "Channel" popup window we can choose any one of the three RGB colour channels. The example shown below is for an adjustment to the red channel. In this case the "midtone" and "highlight" sliders were moved to the left thus increasing red. Moving the "midtone" slider to the right would be the equivalent of increasing cyan. Similarly, if we had selected the blue channel then moving the middle slider to the left increases blue, to the right reduces blue (increases yellow).

 

 

Typically we will want to avoid adjusting the shadow slider, but sometimes, as in our example, it's helpful to move the highlight sliders slightly to reduced colour casts in highlight areas. However, like the "Colour Balance" tool discussed earlier we need to be careful.