A histogram is a visual representation of the brightness values in your image.
Sounds simple enough. And it really is. Once you understand what a histogram represents and how to read it, you’ll wonder why you’ve been avoiding that odd, mountain-range shaped area on the back of your camera or in the corner of your Photoshop screen for so long.
Once you understand how to read a histogram, you’ll have at your disposal an additional and very useful tool for evaluating image exposure. It’s a tool that can improve your photographs, or at least increase the number of keepers that you shoot. That’s because a histogram can point out exposure problems that may not be obvious from the small preview screen on the back of the camera.
By keeping an eye on the histogram, you can see exposure issues more easily and see them while you’re still shooting, instead of later when they’re in full view on a computer screen. The histogram can point out possible exposure issues and give you a hint that you may need to take some additional steps because the contrast in the scene you’re shooting is greater than the camera sensor can handle.
Think of a histogram as an exposure early warning system.
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We've seen how managing the ISO setting allows you to control the amount of grain that shows in your photos. But it does more than that. Understanding and working with the ISO setting gives us added flexibility in terms of setting the other two exposure settings–aperture and shutter speed.
The bottom line is that ISO is an important and useful tool, and one that you will want to be comfortable with if you are looking to take creative control of your exposure.