256 Levels of Light

Ever wonder to yourself what is that little graph on your camera's LCD screen? It's a gold mine of information and it can tell you 256 different things about your image. Well, ok, not exactly. It certainly won't tell you, "Oy! This image sucks. Take it again."However, it will tell you, "Hey! You're overexposed there. Thought you'd outta know", or, "Whoa, dude. You're really dark. Lighten up!"

That little jewel is your image's Histogram.

The image of the old wheel is a perfect example to start off this discussion. It has a very nice spread of lightness and tones. You have your black, gray and white all being represented.

Above is the histogram for the image. How do you read it?

Histograms represent levels of lightness. Black is represented towards the left side of the graph (0), while white is on the right (255). Gray tones are spread throughout the middle in varying degrees depending on lightness levels.

If you look closely, you can see the curve isn't smooth, but rather made up of tiny little jagged bumps. Each bump represents a vertical line marking a tone represented in the image. Each level of lightness is assigned a general number from 0 - 255 (0 being pure black; 255 being pure white; grays go from 1 - 254).

Let's see another example to drive it home a bit more:

This is an image of a tri-tone gray card, which is used to calibrate cameras and white balances. Notice the funky histogram on the right. There's a spike on the left representing the black, a spike in the middle for the gray, and a spike on the right for the white. If this image was all black, you would see a giant spike on the left towards the '0'. Conversely, if this image was all white, you would have a giant spike on the right near 255.

Every image is different, therefore every histogram is different. Sorta like snowflakes.

For example, some images favor the darker side. This is what you call a 'low-key' image.

Just looking at the histogram is enough to tell that this image of the garlic is low-key, as most of the information recorded is bunched up towards the dark side with few light tones showing up on the right.

Now, let's look at the opposite - a high-key image:

Most of the information is gathered on the right side making this a high-key image.

Why should one care about histograms?

Simply put: histograms tell you a bit about the quality of your image. The tangerine photo's histogram is telling me something very important about my image - I have clipping.

The thick bar at the extreme right is on or very close the 255 limit. This means, little to no information has been recorded. It's pretty much pure white, meaning no amount of photoshopping will bring it back. It's gone.

As there's no information in pure white or pure black, any part of your image that falls close to or past zero or 255 will be clipped. For the tangerine photo, I wanted a blown out background with lots of pure white to give it a bright and airy look. But, be that as it may, intended or not, the information was clipped from the image - gone with the wind, never to be seen again.

That's why as I shoot, I frequently check the histogram being displayed on my LCD screen. I can see how my images are looking from an informational point of view. Am I too dark? Am I too hot? A properly exposed image will have all my information safely contained within the 0-255 range of the histogram. I can go into photoshop later and manipulate it to my heart's content.

Well . . . within reason. Histograms are fragile and can be broken. Too much manipulation can result in crappy tones (literally) and noise. One simply cannot decide to turn a low-key image into a high-key image no matter how good the exposure is.

So, heed this little graph.  It is a valuable resource and shouldn't be ignored.