Learn to love your levels

Abril 17, 2007

Demystify those histograms and figure out how to make blacks black, whites white and reduce your mid-tones so your images don’t look quite so washed out.

We can’t all have access to LiseGagne’s studio, Casarsa’s models, or Pattersonminx’s post-processing skills, but there are a few things that everyone should be aware of when processing images, regardless of the content.

One of these is ensuring the contrast of an image is suitable for the subject and concept, so that the tonal range is strong enough to portray the message, as well as ensuring colour and contrast are suitably defined should an image be output to print.

Without the proper contrast, white points turn a dirty grey, while blacks lose their intensity and the mid-tone colours dominate and dull the overall appearance. Just a few blown-out highlights can have serious consequences come printing time. To counter this you could turn to your image application’s brightness or contrast settings, but these will affect the entire tonal range of your image — even those where no adjustment is necessary — while a simpler and more effective method is to look to your levels.

Any graphic application worth using will provide access to an image’s histogram. Photoshop has a dedicated palette that shows the tonal composition of your image at a glance. For many this graph remains a strange and alien concept, but it’s well-worth understanding: every image can benefit from some attention to its levels.

To a computer, a digital image is nothing more than a grid of pixels. Each pixel has 256 potential levels of intensity (ranging from dark to light) for each of the three colour channels — red, green, and blue. Depending on the amount of variation in the intensity of the three channels, your computer is able to put together an image from 16.7 million possible colours, which the human eye deciphers as a recognisable shape, object or scene.

The histogram is simply a graph format that displays the range of intensity contained within your entire image. So if your image is dominated by dark colours you’ll find the graph peaks towards the left, while high-key imagery will find more content to the right. You can also see individual histogram data for each colour channel, although for the most basic adjustment the generic RGB graph is more than adequate.

The information in the histogram is the best way to tell if an image is over or under-exposed. Ideally, with conventional images, you want the graph to extend neatly within the extreme boundaries of the histogram. If you find your histogram has minimal content at either limit, you can be safe to assume that the contrast of your image could be improved, and this is where Levels come into play.

Open the Levels controls (in Photoshop this can be found under Image > Adjustments > Levels) and you’ll find the same histogram presented. Beneath this you’ll find the Output Levels which will generally remain untouched to provide a smooth tonal range of 256 levels (running from 0-255) from intense black to bright white. These Output Levels should ideally equate to the relevant points on the main graph, but if you find data missing from either end then you’ll need to make a small adjustment to ensure your image contrast takes advantage of the full range. Ideally you want to fill the graph as much as possible without touching the extreme ends. Muddied blacks and blown-out highlights especially will lead to undesirable outcomes when it comes time to print.

Look below the main histogram and you’ll see 3 arrow heads — a black one to the left indicates the black point, a white one to the right for highlights and a grey central arrow determining the mid point. Simply click and drag on the black or white control, (depending on the intensity of your image) so this meets the edges of the image’s histogram — you should immediately see a stronger, more dynamic image.

Such a simple approach will quickly correct the average image and make a huge impact on the contrast range making your images much more appealing to buyers. More complex images, however, may have a less simplistic histogram to adjust and the novice may find the manual approach presents new challenges. The Auto-Levels option also found under the Image > Adjustments menu may take much of the guesswork out of things for a quick fix, although if you return to the Levels dialogue you’ll find a few other options to help out. If you use Auto-Levels, always double-check to make sure that the computer got it right.

Once you’ve experimented with basic levels adjustments you’ll start to appreciate the power available through this method of correction. Editing individual channels is an effective way of balancing colour content, boosting colour content or emulating different processing styles. When you get comfortable with reading the histogram, checking the graph on the back of your camera will become a valuable way of adjusting your exposure during a shoot. Remember, it’s always best to get it right in-camera.

Levels also provide a powerful method of burning-out white backgrounds or intensifying black backgrounds if you have isolated subjects for example. A quick fix for this can be created by making a feathered selection to contain the background and then making the relevant Levels adjustment just over the background, so the contrast of the primary subject remains unadjusted. More effective methods may be achieved over more complex objects using masks, curves and adjustment layers but they’re best left for another article…

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