Understanding Histograms:
Pixels from Dark to Light

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Understanding histograms is a fundamental throughout the digital photography workflow.

Histogram is the name given to a two-dimensional graph showing the quantity and distribution of pixels within a digital photograph.

You may have seen them right at the start of the digital photography process whilst scrolling through the different display modes on your camera.

Further down the line, in applications such as Lightroom, Aperture and Photoshop, they just keep showing up!

At first sight, understanding Histograms looks technical and perhaps complicated but, don't worry, it's not.

So what does the histogram mean and why do we need it..?

Firstly, the Histogram shows us the total quantity of pixels in a photograph.

Secondly, we know that a photograph generally contains:

- Dark pixels, or shadows;

- Light pixels, or highlights;

- Everything else in between, or midtones.

So, on the left of the graph are the darker pixels, the shadows...

...and on the right are the highlights:

In between, you guessed it, are the midtones:

If the furthest pixels on the left don't reach the end of the scale, leaving a gap, then there are no true blacks in the photograph.

Similarly, if there is a gap on the right, there are no true whites (not necessarily a bad thing by the way!):

Taking this a step further, if the pixels are ramped up against the left, there are true blacks in the photograph.

In this case, shadow detail has likely been lost forever (or not even captured in the first place).

At the other end, if pixels are all bunched on the right, there are true whites in the file.

This time, highlight detail has likely gone (or not captured), never to be seen again:

The latter is often most apparent in subjects such as white, fluffy clouds. Another example is hot spots on skin tones...

The technical term for this loss of detail at the shadow and highlight extremities of the scale is known as clipping.

Again, this is not necessarily bad but is generally less desirable at the highlight end of the scale where clipping can be unsightly.

For the moment, the last thing I would like you to consider is that the scale stretches from 0 (the darkest possible value on the left) to 255 (the lightest possible value on the right).

0 to 255, that's 256 tones (or shades) altogether.

Whoa! That's quite a lot of information to digest in one hit!

Don't get too bogged down by Histograms though. They will always be a useful tool to assess your masterpieces but, as someone once said:

"Remember, nobody's hanging Histograms on the gallery wall!"

There's more to learn but that's all I want to cover for the moment in this initial overview.

In the meantime, more tips on understanding Histograms crop up throughout the site - for example, in this Levels Tutorial.

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