thinking outside the box
Photo Editing Guide Part 3 - The Enhance Options
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Brightness Levels Histogram

A Brightness Levels Histogram is basically a plot of the brightness levels of all the pixels in the image, from the darkest on the left to the lightest on the right. The more pixels that are a given brightness, the higher the chart value. It’s a great way to see the distribution of bright- ness levels of your picture, at a glance, and to see the effect of these controls on the brightness distribution. As you adjust any of the brightness controls, the histogram changes, showing you the original levels (the darker red area in the above case) and then overlaid this a lighter semi-transparent adjusted levels (the lighter pink in the above example). So this means as you adjust the controls you get instant feedback of how your brightness levels distribution is affected. In the following examples, each photo is shown with the results of the enhance operation, and under each is the histogram of the brightness levels with an explanation. Of course, you do not need to use or even see the Brightness Levels Dialog (described below on the last page of this tutorial) and the histogram - you can just adjust the controls until the picture looks right to your eyes. But it’s educational to understand and see a visual graph of the changes.

Brightness Control

The brightness control adjusts the overall brightness of the image, but unlike the brightness control in most photo editors that brighten all values equally, this puts greater emphasis on the darker shades in your picture. The green line represents how the brightness levels are adjusted. In the initial image (left example) there is no adjustment - it’s a straight line. In the center image where the brightness value is increased you can see that the left end (darker shades) are lifted, but the brighter shades (right end of the line) are hardly changed - the line is very close to the original. In the case of the darker right-hand image, the green line curves downwards indicating that all the shades have been made darker.

Contrast Control

Increased contrast means making the lighter shades lighter and the darker shades darker. Reduced contrast is the opposite. At extreme low contrast, almost all the image is mid-tone gray.
The original image - the brightness histogram shows largely mid-tone shades with a peak at the white end. The green line ‘levels adjustment’ is an unaltered straight line. With increased contrast the green line shows the adjusted map. The darker shades (to the left) have been made darker, the brighter shades (to the right of the center point) have been made brighter, resulting in a slightly S shape curve. With reduced contrast, darker shades are made brighter, and brighter shades made darker. The resulting histogram reflects this, so the range of brightness tones is now much narrower (the overlaid pale red peak is narrower, now concentrated in the mid grey range). Original image Contrast +29 Contrast -45
Original image Brightness +12 Brightness -21 A preponderance of mid-brightness values in this image, with a peak at the white end. When you adjust the brightness levels you see two charts on top of each other. The original (darker red) and the adjusted (paler red, semi-transparent). The dark peak shows the original brightness distribution - same as the left example. The overlaid semi-transparent pale red peak shows the adjusted brightness distribution, which for this brightened image is shifted right - towards the brighter shades. The overlaid pale chart shows the adjusted brightness distribution, in this case moved left, so most of the shades have been made darker. Note the white peak in both cases remains untouched.