ADJUSTING LEVELS TO IMPROVE CONTRAST
Prerequisites: Understanding histograms will make it easy to use the levels tool.
Levels is a simple tool available in most photo-editing software for improving contrast of the image. Using levels can increase the contrast to make the image look rich with saturated colors. In Photoshop, levels tool can be invoked by Image >> Adjustments >> Levels.
The levels tool shows the histogram with three sliders at the bottom of it, as you can see below.
Above is the levels display for the image shown below.
ADJUSTING BLACK POINT
The black slider to the left of the histogram(marked '1') determines the black point of the image, where all RGB values are zero. By shifting the black point to the right, all pixels to the left of the slider will be made completely black(RGB values zero). The histogram will be re-adjusted to reflect the new black value, resulting in reduction of overall brightness of the image. Note that this is not same as reducing the brightness using Brightness/Contrast tool. While Brightness /Contrast tool uniformly reduces the brightness of the image, black point slider of the levels tool affects darker portion of the image more than brighter portions. It also does not result in any shift in the white point. That is, all pixels that are completely bright(RGB values 255) remain unaffected. You can see the effect of moving the black point slider on the image below.
See that the image turns darker. Also see the input levels on the top, which indicates that the pixels with brightness 19 and below will become zero.
ADJUSTING WHITE POINT
The white slider to the bottom right of the histogram(marked '3') adjusts the white point of the image. Moving the white slider to the left results in all pixels to the right of the slider to become full white(RGB levels 255). Similar to moving the black slider, histogram will be re-adjusted to reflect the new position as white value(255), resulting in the overall brightness of the image. Again, this adjustment affects brighter parts of the image more than darker parts, and leaves the black point unaffected. See the effect of adjusting the white point below.
ADJUSTING MID-TONES
The middle slider(marked ‘2’) is used for adjusting mid tones(brightness level 128). For example, if you move the slider to the right, mid tones get shifted to the new position to the right, rendering the pixels to the left of the slider to go below the mid-tone value and hence reducing the brightness of the mid-tones. Moving the slider to the left results in pixels to the right of the slider go above mid-tone value, resulting in increase in brightness of the midtone. This results in change in the brightness of the mid tones without affecting black point and white point.
Here, I have moved the midtone slider to the right, resulting in reduction in midtone brightness. The ratio of new mid-tone value to the old-value is often called as 'gamma'. The middle text box above shows the value of gamma as you move the slider. In this case, it indicates that the new mid-tone has a brightness level that is 0.83 times the initial brightness.
INCREASE IN CONTRAST
Note that the changes made in the levels has improved the contrast of the image. See the comparison of the image before and after applying levels.
When adjusting levels, it is recommended to shift the black slider to the right to a point until the graph starts rising, and white slider to the left to a point until the graph starts rising. This gives the best contract without loosing any color information. After doing this, adjust the gamma to either side until the image looks at its best. This is exactly what I have tried to do above.
PREVENT CLIPPING OF COLORS
To ensure that no color information is lost because of clipping when moving black or white sliders, hold the Alt key when moving the sliders. On holding Alt key, the entire image look white. And as you move the slider, any clipped colors will start appearing on the screen. To ensure that no color information is lost, stop the slider at a point before you start seeing any colors on the screen.
ADJUSTING INDIVIDUAL COLORS AND BRIGHTNESS
The above adjustments were made on the overall image brightness. Level adjustments can also be made on single color channel by choosing the drop-down box on the top. However, adjusting levels of individual colors is a job for experts and is best not done, as it may result in color shift in the image.
The levels tool can also be used to for color correction of the image by using the eye-dropper tools seen in the bottom right of the tool. But this deserves a special attention, and can be a separate chapter by itself.
Levels is a simple tool available in most photo-editing software for improving contrast of the image. Using levels can increase the contrast to make the image look rich with saturated colors. In Photoshop, levels tool can be invoked by Image >> Adjustments >> Levels.
The levels tool shows the histogram with three sliders at the bottom of it, as you can see below.
Above is the levels display for the image shown below.
ADJUSTING BLACK POINT
The black slider to the left of the histogram(marked '1') determines the black point of the image, where all RGB values are zero. By shifting the black point to the right, all pixels to the left of the slider will be made completely black(RGB values zero). The histogram will be re-adjusted to reflect the new black value, resulting in reduction of overall brightness of the image. Note that this is not same as reducing the brightness using Brightness/Contrast tool. While Brightness /Contrast tool uniformly reduces the brightness of the image, black point slider of the levels tool affects darker portion of the image more than brighter portions. It also does not result in any shift in the white point. That is, all pixels that are completely bright(RGB values 255) remain unaffected. You can see the effect of moving the black point slider on the image below.
See that the image turns darker. Also see the input levels on the top, which indicates that the pixels with brightness 19 and below will become zero.
ADJUSTING WHITE POINT
The white slider to the bottom right of the histogram(marked '3') adjusts the white point of the image. Moving the white slider to the left results in all pixels to the right of the slider to become full white(RGB levels 255). Similar to moving the black slider, histogram will be re-adjusted to reflect the new position as white value(255), resulting in the overall brightness of the image. Again, this adjustment affects brighter parts of the image more than darker parts, and leaves the black point unaffected. See the effect of adjusting the white point below.
ADJUSTING MID-TONES
The middle slider(marked ‘2’) is used for adjusting mid tones(brightness level 128). For example, if you move the slider to the right, mid tones get shifted to the new position to the right, rendering the pixels to the left of the slider to go below the mid-tone value and hence reducing the brightness of the mid-tones. Moving the slider to the left results in pixels to the right of the slider go above mid-tone value, resulting in increase in brightness of the midtone. This results in change in the brightness of the mid tones without affecting black point and white point.
Here, I have moved the midtone slider to the right, resulting in reduction in midtone brightness. The ratio of new mid-tone value to the old-value is often called as 'gamma'. The middle text box above shows the value of gamma as you move the slider. In this case, it indicates that the new mid-tone has a brightness level that is 0.83 times the initial brightness.
INCREASE IN CONTRAST
Note that the changes made in the levels has improved the contrast of the image. See the comparison of the image before and after applying levels.
When adjusting levels, it is recommended to shift the black slider to the right to a point until the graph starts rising, and white slider to the left to a point until the graph starts rising. This gives the best contract without loosing any color information. After doing this, adjust the gamma to either side until the image looks at its best. This is exactly what I have tried to do above.
PREVENT CLIPPING OF COLORS
To ensure that no color information is lost because of clipping when moving black or white sliders, hold the Alt key when moving the sliders. On holding Alt key, the entire image look white. And as you move the slider, any clipped colors will start appearing on the screen. To ensure that no color information is lost, stop the slider at a point before you start seeing any colors on the screen.
ADJUSTING INDIVIDUAL COLORS AND BRIGHTNESS
The above adjustments were made on the overall image brightness. Level adjustments can also be made on single color channel by choosing the drop-down box on the top. However, adjusting levels of individual colors is a job for experts and is best not done, as it may result in color shift in the image.
The levels tool can also be used to for color correction of the image by using the eye-dropper tools seen in the bottom right of the tool. But this deserves a special attention, and can be a separate chapter by itself.
posted by Arun at 9:47 PM
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