Image Histograms: What are they, and why are they important for digital images?

IMAGE HISTOGRAMS

Image histograms are ubiquitous to digital images. You’ve probably seen them on your camera or any photo editing software. In this tutorial, I will be showing screen shots of image histograms from Adobe Photoshop

How do I find the image histogram in Photoshop?
To find the histogram that shows the various RGB colors, go to Window (top navigation bar) > Histogram
To find the histogram that shows the various brightness values of the colors, go to Image>Adjustments>Levels
**I will do a tutorial following this one that describes how to use Levels**

Basics:

What is an image histogram?

Every color in a digital image has a brightness value ranging from 0 (black) to 255 (white).
An image histogram is a histogram of brightness values of every pixel in an image

Tones and Tonal Ranges

The tonal range is the region of the histogram were most of the brightness values are present.

Image
This is a standard histogram of brightness values
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This is a color histogram

What can an image histogram tell you about your image?

Lighting: is my image High Key or Low Key?

High Key Image: an image in which most of the tones are in highlights

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Notice how most of the pixels are found in the highlight region of the histogram
ImageImage

Low Key Image: an image in which most of the tones are in shadowsImage
Notice how the pixels are mostly found in the shade region of the histograms
ImageImage

Overexposure of image: is my image overexposed?

An overexposed, aka clipped, image may have regions that are overexposed to the point that they are solid white and no detail can be seen.
Notice how the top left corner of this image is clipped.

Image

This image’s histogram will show that the brightness values for some pixels are pushed to the very edge of the chart:
ImageImage

Breadth of Tonal Range: How many tones are in my image?

An image that has a broad tonal range will have a high pixel count on the far left and far right side of the histogram.

Image

ImageImage

An image that has a narrow tonal range will have a high pixel count in one region.

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Most of the colors in the picture are a light yellow or pink.. this is why the histograms have only 2 main peaks.
Notice that this picture also has high-key lighting
ImageImage

Amount of contrast in image: How much contrast is in my image?

A histogram with a broad distribution of brightness values also tells you that there is high contrast in the image.

Image
ImageImage

On the other hand, a histogram with a narrow distribution of brightness values also tells you that there is low contrast in the image.
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Notice that this image also has low-key lighting.
ImageImage

I hope this tutorial was useful to you!
Now that you have knowledge of how to read an image histogram,

learn how to manipulate the histogram by using Levels in my next tutorial!

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