Brightness and contrast adjustments are among the most common functions performed on digital images. Because of this – and because photographers expect to be able to fine-tune these adjustments – most editors provide several ways of making these adjustments.
The Brightness/Contrast sliders in Photoshop.
The simplest tool is a basic pair of Brightness/Contrast sliders. This is quite a crude tool because the adjustments act equally across all tones in the image. There is an ever-present risk of blowing-out highlights while you attempt to bring up details in shadowed areas.
Most editors include an Auto Contrast adjustment, which identifies the brightest and darkest tones in the image and attempts to spread the intermediate tones evenly across this range. The end result will be largely dictated by the spread of tones in the original image. It works best with images that have evenly distributed tones but is less successful with high-key (predominantly light) or low-key (predominantly dark) images.
While automatic adjustment tools can do the job, experienced photographers tend to make manual Levels adjustments via the Levels tool, which normally resides in the same drop-down menu as the Brightness/Contrast sliders. Different image editors may place the Levels setting in different drop-down menus. But they work in the same way in all editors.
A typical Levels adjustment dialog box.
The histogram display that dominates the dialog box plots tones from pure black on the left to pure white on the right along the horizontal axis. The vertical axis plots the number of pixels at each tonal level for 256 steps (0 to 255) on the horizontal axis, giving you a picture of where the highest and lowest concentrations of tones lie.
Above the histogram are two dropdown menus: Preset and Channel. The Preset menu may contain some pre-determined adjustments, most of which can be safely ignored. You can save your own pre-sets here, based upon the values for typical images in a collection.
The Channel menu accesses the”¨ three colour channels (red, green and blue) and has a default setting of RGB, which adjusts all three together, thereby controlling global tonal values in the image.
Above the histogram are the words ‘Input Levels’. These can be adjusted with the three sliders (shown by arrows) on the base line of the graph. The left hand slider adjusts the shadow areas in the image, while the right slider adjusts the highlights, leaving the middle slider for adjusting the midtones.
The levels dialog box, shown with an image in the Photoshop workspace.
Below the histogram is a grey scale labelled ‘Output Levels’ with sliders at each end. By default, when the dialog box is opened, the Output sliders are at level 0, where the pixels are black, and level 255, where the pixels are white. With the Output sliders in the default positions, moving the white point slider maps the pixel value at its position to level 255. Moving the black input slider maps the pixel value at its new position to level 0.
If, for example, you move the white point slider to the left as shown in the illustration below, all the pixels at level 202 are mapped to 255 (the white point). Similarly, moving the black point slider to 5 maps all pixels at level 5 to level 0.
Moving the white point slider in to the edge of the graph maps the pixels at level 202 to 255, making the image appear brighter.
This mapping affects the darkest and lightest pixels in each channel and all colour channels are adjusted proportionately to avoid shifting the colour balance. The remaining levels are redistributed between these levels, which increases the tonal range in the image. This has the effect of increasing the overall contrast.
Moving the middle Input slider to the left brightens the mid-tones in the image without dramatically altering the highlights or shadows.
Moving the middle Input slider changes the midtone from a default value of 128, thereby adjusting the intensity values of the middle range of tones without dramatically altering the highlights or shadows. In Photoshop, you can identify areas in the image that will be clipped (completely black or completely white) by holding down the alt key (Windows) or Option key (Mac OS) as you move the black point and white point sliders.
Care must be taken when adjusting the Levels for high-key images. The middle slider can provide some degree of tonal control that preserves the essence of the shot.
While the adjustments described here should work well with most images, there are always exceptions to the rule. High-key images will generally need to have histograms that are weighted towards the right, whereas low-key images will be weighted to the left. Images with low contrast need particular care if the low contrast is to be preserved. The middle slider can be particularly helpful in such situations.
The Curves control
The Curves control harks back to the characteristic curve that described tonal distributions in black and white films. Although digital image sensors tend to produce straight-line tonal distributions, the Curves control can be used to make small (but visually significant) adjustments to shadow and/or highlight areas.
The Curves function plots the tonal histogram against a diagonal line, which is used for increasing or reducing brightness in a particular tonal zone.
The bottom axis of the graph shows shadows at the left hand end and highlights at the right.
Midtones lie between them. The vertical axis represents lightness of the tones.The tonal curve is controlled by anchor points, which are set up by clicking on the diagonal line. Dragging an anchor point upwards increases the brightness of the selected tone, at the same time pulling up adjacent tonal points. Lowering an anchor point darkens the tone and surrounding areas.
Points can be introduced anywhere along the line, giving a high degree of control because you can place them close together or far apart. When points are not moved, they anchor the tone to the original tone recorded.
This illustration shows four anchor points along the curve for a contrasty original with slightly over-exposed highlights and deep shadows. Moving the top anchor point (circled) down darkens the extreme highlights and also pulls down other highlight areas. Moving a lower anchor point (circled) up brightens the shadows. The second lowest anchor point has also been raised slightly to assist the shadow brightening. The central point remains in place to fix the mid tones to those in the original image.
Curves can be used to manipulate contrast. Dragging a point in the lower third of the diagonal line down and raising a point in the upper third slightly will create an S-shaped curve that subtly increases contrast. Doing the reverse (dragging the low point up and the high point down) reduces contrast. In each case, these adjustments should be subtle; large changes in point position can result in blown-out highlights and blocked shadows.
It’s important to avoid raising a darker point higher than a lighter one (or lowering a bright point below a brighter one). Doing this upsets the tonal balance in the image and will produce unnatural-looking results.
This illustration shows how tones can be distorted when a darker point is moved highter than a lighter one (circled) on the Curves control. note the unnatural colours in the shy in the top left corner and on the lit side of the windmill.
Some editors include a Shadows/Highlights adjustment, which can be used to bring out details in the shadow and highlight areas of an image that in the original were too dark or too light to see. Selecting this function applies some adjustments automatically.
Selecting the Shadows/Highlights adjustment in Photoshop.
The main advantage in using the Shadows/Highlights command is the adjustment options it provides. As well as sliders that control the amount of the change applied, there are also sliders for adjusting the tonal width and the radius over which changes are made.
The range of adjustments available in the Shadows/Highlights dialog box. (Compare the auto-adjusted image here with the original image shown in the background of the screen grab above).
The Tonal Width slider sets the range of tonal values that will be affected by the adjustment. At its lowest setting, only the darkest areas of the image will be affected. Dragging the slider to the right will include more of the mid-tones. Different images will require different degrees of adjustment so you need to experiment each time and choose a setting that looks best.
The Radius slider determines how the adjusted areas of the image will blend in with the rest of the photo. Setting the slider too far to the left can produce harsh transition areas between the adjusted and unadjusted areas of the image and will often make the image appear lacklustre. For most images, a radius value between 25 and 75 pixels (px) will work best.
If after adjusting the shadows and highlights you find your image has lost some of its original zap, the Colour Correction slider can give the saturation a boost, while the Midtone Contrast slider can be used to increase contrast in these values to improve the look of the image.
We’ve demonstrated these adjustments on JPEG files for the purpose of simplicity. But to achieve the best end results, we recommend using raw files from the camera, which have been converted into editable TIFF (tagged Image File Format) ““ preferably with 16-bit depth.
Excerpt from Photo Editing Pocket Guide