Just before we get onto histograms, let's look at a few other terms related to a concept called tonal range.

Tonal Range, Shadows, Midtones and Highlights

Tonal range is a term used to describe the range of brightness levels in an image from the darkest to the lightest.  For many pictures (but not all) you want this to cover the full range possible from jet black to brilliant white.  If it does not, then the image can look flat.  Look at these two black and white images - the one on the right has the greater tonal range as it has blacker blacks and whiter whites.

     
Photographers use the terms shadows, midtones and highlights when talking about different parts of the tonal range.  The darker parts of the image are referred to as the shadows, the lighter colours as highlights, and the ones somewhere in the middle (mid-greys) are the midtones.  Some of the tools you will come across in Photoshop allow you to apply the effect / brushstroke to just one of these three regions of the tonal range.  For example to create a more dramatic black and white landscape, you may want to selectively darken just the shadows.

A histogram is a visual way of representing the tonal range of an image.  It can be used to show you whether the image is under or overexposed and whether you are exploiting the full tonal range that’s possible in the image.  What it can’t do is tell you whether the image looks good as sometimes an image needs to be over or under exposed to create the desired effect.  So use histograms as a guide to help you and not to govern all the decisions you make.

A histogram is a graph that shows you the full tonal range along the horizontal axis and an indication of the number of pixels at that point in the range by the height of the graph.  When you have an image loaded in Photoshop or Paint Shop Pro, you can view the histogram by turning on the Histogram Palette.

Here are some examples to get you used to looking at them:

	This histogram shows a reasonably good tonal range.  There is a gap at the top (shown in red) indicating that there are no bright whites in the image but this can easily be corrected with the levels tool covered later on.
	This histogram shows a gap at the dark end of the tonal range indicating a lack of deep blacks in the image.  It also shows a peak at the right edge - a sign that some of the highlights have turned completely white (burnt out) and therefore lost all detail.  This is a typical of an image which has been over exposed.
	This one shows a gap at both ends of the histogram meaning that the scene was probably quite low in contrast to start with.  Although this might accurately show what was taken, you can stretch out the histogram to cover the whole tonal range to give a more vibrant image (as shown in the boat pictures on the previous page).

The histograms on the previous page show the range of luminance values in the image.  Although this is great for checking the general exposure of an image, it is sometimes useful to be able to look at the histograms for individual colour channels.  All good imaging software should enable you to do this.

The image on the right shows the separate RGB histogram from Adobe Camera RAW (part of Photoshop).  As all three are shown on one graph, the combined colour is shown where two overlap, and white where all three overlap.  The image below it shows the same RGB histograms as they are displayed in Paint Shop Pro which has the added flexibility of being able to display histograms for hue, saturation, lightness and greyscale (another name for luminance). Regions where an individual colour channel have become clipped lose all texture in that colour.  However if the other 2 channels have not been clipped, some luminance texture may remain.