Sophie Elizabeth Lynch: Exposure Histograms for Over, Under & Correct Exposure

How to use Histograms

The best way to evaluate exposure is to look at the picture, not a histogram. Histograms are a way to measure exposure more objectively for those who can't see very well. Histograms don't replace your eyes and experience. Histograms are helpful in sunlight where it's hard to see an LCD, or in the shop if setting something exactly. Your eyes are always the final judge.

A histogram is a graph counting how many pixels are at each level between black and white.

Black is on the left. White is on the right.

The height of the graph at each point depends on how many pixels are that bright.

Lighter images move the graph to the right. Darker ones move it to the left. Easy!

More Pixels Fewer Pixels
	Black - Dark - Medium - Light - White A Histogram

A good image often, but not always, has a histogram spread all over.

SETTING EXPOSURE
Showing a single histogram to simplify. DON'T use a single histogram to set exposure! You need a colour histogram, otherwise you may overexpose coloured areas and not know it. Read on to Colour Histograms after you read this.

Contrary to your camera manual, the histogram doesn't have to be in the middle. Black cats in coal mines may only use the left half. Snow scenes may only use the right half.

The critical thing for which a histogram is helpful is to determine if any highlights have been clipped and washed out. Overexposure is death for a digital image. Histograms make this easy to check. If you have washed-out areas of 100% white (digital value 255) you'll see a tall vertical line at the far right of the histogram.

If you blow an image to smithereens you'll see more than just one line peaked on the right. You may see a train wreck!

In these three photos I took you can see that I took them using three different settings on my camera.

Firstly I changed the camera to 'S' which is 'Shutter Priority Auto', and changed the numbers where is circled in red to '0.0', '+0.3', and '-0.3'.