The histogram displays the amount of blacks, whites and neutral tones in an image. By reviewing your histogram after each shot you will gain a better understanding of how the camera processes the scene and be able to make any adjustment to achieve that perfect exposure.
In the sample of the lighthouse image, you will notice the histogram in the top. On the far right of the histogram, this is the number of whites, across the middle are the neutral tones, and the far left is the blacks.
You will notice where the “1” is on the histogram, this is showing the white pixels displayed in this image. If this was all the way against the histogram on the right side the shot would be overexposed and you would lose detail in the whites. If the furthest right spike in the histogram was more to the left the image would be underexposed. Since we have white in this image, the histogram should have a spike at the furthest possible right, without touching the side of the histogram.
“2” is showing the blacks in the image. If this was too far left and spiked against the wall of the histogram, the image would be underexposed and detail would be lost in the blacks. If the furthest left spike was more in the middle for this image, the image would be overexposed. Since we have black in this image, the histogram should spike at the furthest possible left, without touching the side of the histogram.
The “3” is showing the neutral tones in the grass and sky.
“4” is the darker trees.
By reviewing your histogram after each shot, and identifying where the spikes are, you can then make adjustments to your images correcting the exposure .
Below is another example of an image correctly exposed, under exposed and over exposed so you can compare the visual appearance of the photo and how the histogram displays the exposure. Notice the placement of the data within the histogram and remember to avoid those large spikes at the very left and right edges to keep detail in your whites and blacks.