Digital Photography / Photoshop
Histogram
Histogram
The histogram in Photoshop graphs tones in your image.
Tones are from 0 on the left to 255 on the right. Zero is black (darkest point) and 255 is white (brightest point). Tones on the right side of the histogram are highlights in the image and tones on the left side are shadows. The number of pixels in each tone is height on the histogram.
Midtones are controlled by the gamma (center) slider. The gamma slider shifts midtones without changing highlight and shadow points.
You can view a histogram of an image in the LCD on the back of your camera right after taking the shot to evaluate the quality of the exposure, then correct the exposure and retake the shot if necessary.
In Photoshop you can adjust the tones of an image using the Levels histogram. You adjust the tones by adjusting any of the three sliders beneath the graph as described above.
Example 1
When you open Levels, you see the histogram for 004 graph skewed to the left. If you want to just check the histogram, you can open Histogram from Window menu.
More pixels are in the dark tones than in the light tones.
You want to spread out the tones so that they are equally distributed in dark, middle and light areas of the histogram.
You do this by moving the white slider until it touches the curve.
By moving the white slider to the edge of the curve, the right edge of the curve becomes the brightest point (255) of the image and the entire curve spreads out to form a new curve that is evenly distributed from 0 (black) to the new 255 (white). Pixels at the dark side have shifted toward the brighter side of the curve and are now more evenly distributed over the entire range of the image.
Example 2
When pixels are bunched toward the right side or bright end of the histogram as in Beach,
you can try moving the black slider to the right until it meets the curve.
The tree trunk looks better but the image is still overexposed.
You can move the middle (gamma) slider to the right effecting only the midtones.
But this only darkens the tree stumps. The image is still overexposed. Overexposed images are difficult to correct.
It is best to make sure the image is not overexposed by checking the histogram in the camera LCD after taking the picture. By insuring that this histogram does not show any (or at least very little) bunching at the right side, you can avoid overexposure.
Example 3
But the histogram doesn' t tell everything about the image.
If you were to look at the histogram for this orchestra image,
you would conclude the image is underexposed.
Most of the pixels in the image are bunched toward the black side of the histogram. But you can see this image is acceptable as is.
