Features

It may sound geekily technical, but 'histogram' is just a fancy word for a bar chart. In image editing, the histogram is a standard type of display that shows how many pixels in your photo correspond to each level of lightness, from minimum (black) to maximum (white).

OK, that probably doesn't sound much clearer. Well, each pixel in a digital image has a colour value, which is stored by recording the amount of red, green and blue light at that point. Add up all three values and you get a number that tells you nothing about colour, but indicates how bright the pixel is: with no red, green or blue light it'll be totally dark (black), and with the maximum amount of all three it'll be pure white.

Counting up how many pixels correspond to each point along a scale from dark to light may seem like a pointless academic exercise, but in fact it tells you a lot about the photo's content, and can help you identify problems. An image containing lots of shadow will have more pixels at the dark end of the scale; lots of bright areas and they'll be biased towards the light end. If few or no pixels are at the dark or light ends of the scale, the image lacks deep shadows and/or bright highlights, which will tend to make it look dull.

By presenting the pixel counts on a graph, your image editing software can show you all this stuff at a glance - you just need to know how to read it. And even better, by attaching interactive controls to the graph, it can let you squash and stretch the pixel values to correct problems or alter the appearance of the photo to your liking. This works far better than Brightness/Contrast controls.

Levels versus Curves

An alternative way of editing an image's pixel values is to apply a tone curve. This is done using Curves in the full version of Photoshop; a similar command is provided in some other image editors, including Corel Paint Shop Pro Photo, but not in Photoshop Elements. Instead of dragging markers, you add points to a line on a graph that plots input (the existing pixel values) against output (the result you're creating). A widely used pro retouch trick is to apply an 'S curve', as seen here. This will make almost any photo look more vibrant and punchy.

	1 The best place to view and interact with the histogram is in the Levels dialog box, available in all good image editors. To open it in Photoshop Elements, press Ctrl+L. The graph display consists of narrow black columns representing numbers of pixels, from the darkest colour levels (shadows) to the lightest (highlights), left to right.

	2 This photo was taken on a bright, sunny day in the middle of summer, using a worthy digital camera (a Panasonic Lumix DMC-FZ50). What went wrong? The sun is so bright here that much of the subject is thrown into deep shadow. Overall, the image is well balanced, with good contrast -but it looks poor because little detail is visible.

	3 The histogram illustrates the problem: most pixels are clustered at the dark end of the scale. Now to fix this. Drag the grey marker (below the histogram) to the left: this bends the scale so more pixels get lighter values. You can also drag the black Output Levels slider a little to the right, bringing the darkest pixels away from pure black.

	4 Click OK. The shadows are tamed, but now the photo looks a bit washed out. To boost contrast, open Levels again, and drag the black and white Input Levels markers a little way towards the middle. (This will also work on photos that look dull to begin with; the Auto Levels option has a similar effect.) When you like the result, click OK.

	5 Before The house in this photo is lost in shadow, but using Levels you can reveal it. With different settings, the same command is just as good for rescuing photos that have too little rather than too much contrast. Get to know how it works, then judge the best settings for each shot by eye.

	6 After