what is a histogram
understanding how to use a histogram

You've heard them mentioned several times, but what is a histogram and what do you need to know about them?  

For much of the digital photography we do, we really don't need to look at an image's histogram. Digital cameras always get the exposure just right. It's auto-exposure, correct?

Well, usually, but not always.

There are certain situations when histograms are extremely valuable to. Histograms are great tools for combining advanced technology with practical thinking.

Histogram used for outdoor family portrait.Histogram revealed clipped highlights - but they were of the white wall in the sunlight behind my subjects.

A basic understanding of what they are and how histograms are used will help you improve you ability to get the right exposure when your camera's auto-exposure fails you.

The simplified definition of a histogram: 

A digital photography histogram is a graph that shows us the relative number of pixels from the darkest shadow areas of a photo to the brightest areas in that photo.

Digital camera meters and their built in processing software do a great job of estimating the correct exposure most the time. When you have an unusual subject or unusual lighting situation, these graphs are very valuable.

Unusually dark or light subjects as well as situations of very high or very low contrast dictate the use of using a histogram.

The two major warning signs show when the dark areas of the graph press against either side of the histogram. These are the two most common red flags that indicate a need to adjust the exposure.

Diagram of histogramsComparison of three histograms for three different exposures

You can see the three examples above with the snapshots of my dog "Mocha" sleeping on my office floor. The histograms for each photo are directly below them.

In the left photo, it is clearly under exposed because the graph underneath that dark photo show a significant amount of the black area is touching the left side of the graph.

If we kept the exposure at this level, we would lose details in the shadow areas and brightening when we do our photo editing would reveal lack of detail and too much digital noise in the edited photo.

The middle photo is properly exposed. The photo on the right is overexposed. It looks "about right" but if we look at the histogram we can see that the graph has an area that is against the right side of the graph. In this situation we have lost details in the highlight areas that are lost forever and can never be fixed with editing afterward.

Histogram of dark-haired dog

This photo of Mocha is perfectly exposed.

The histogram shows a large amount of dark pixels in the photo.

This leads the biggest photo mistake that many photographers make.

At quick glance, one might think the image is underexposed.

Most of the tones in this photograph are dark, so the histogram will have most of the area of the photograph on the left side of the graph. It is not under exposed.

The highlights, although they are few in number are properly showing at the right end of the histogram. There are also no black areas in the graph that is touching the left side of the histogram.

Once you get a basic understanding of them, digital photography histograms can be analyzed with a quick glance and you can adjust your exposure up or down in an instant.

The best way to learn more about them is to look at your histograms after taking several different photos with different subjects. You can have some fun by guessing ahead of time what your graph will look like.

You will be surprised how much this will help you in those situations that need extra attention in getting the correct exposure for your digital photos.

what is a histogram in color

The histograms above are the simplest and most commonly used for the majority of photographers.

This histogram is from a photo of a red flower.

Color histograms are a bit more sophisticated.

They show pixels for the three different color groups that make up our visual spectrum, red, green, and blue light.

With subjects that are highly saturated with one of the three color groups, one of the colors may actually be overexposed.

The color histograms shows the overexposed red pixels that are not revealed in the simple combined histogram.

Histograms can reveal color imbalances

Histograms can reveal color imbalancesHistogram revealed tungsten color setting mistake

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