June Lesson: Histograms

[Diane Margetts]

HISTOGRAMS

 

Histograms are a very useful tool that many cameras offer their users to help them get a quick summary of the tonal range present in any given image.

It graphs the tones in your image from black (on the left) to white (on the right).

The higher the graph at any given point the more pixels of that tone that are present in an image.

So a histogram with lots of dark pixels will be skewed to the left and one with lots of lighter tones will be skewed to the right.

 

 

The beauty of a histogram is that the small LCD display on your camera is not really big enough to give you an great review of a picture and you can often get home to find that you’ve over or under exposed an image. Checking the histogram can tell you this while you’re in a position to be able to adjust your settings and take another shot. CHANGING ANY OF THE ELEMENTS IN THE EXPOSURE TRIANGLE--APERTURE, SHUTTER SPEED OR ISO, WILL ALTER THE HISTOGRAM BY CHANGING THE EXPOSUR OF THE PHOTO. TWEAK THE SETTING TO GET A BALANCED HISTOGRAM.

 

The actual histogram graph looks like a mountain peak, or a series of peaks. The more of a particular color, the taller the peak. In some cases the graph will be rounder on top, or flattened. The left side of the histogram represents the maximum dark values that your camera can record. The right side represents the maximum white values your camera can capture. On either end of the histogram the light values contain no detail. They are either completely black, or completely white. The top of the histogram (top of mountain peak) represents the number of different colors, a value you cannot control, so it is for your information only. We are mostly concerned with the left and right side values of the histogram, since we do have much control over those. (Dark vs. Light)

 

So, basically, the histogram's left to right directions are related to the darkness and lightness of the image, while the up and down directions of the histogram (valleys and peaks) have to do with color information. I repeated this for emphasis! The left (dark) to right (light) directions are VERY important for your image making. If the image is too dark, the histogram will show that by clipping off the light values on the left, or, if too light, by clipping on the right. This will become easier to understand as we look at well exposed and poorly exposed images. Check out the Basic Tutorial Below, then we'll look at things in more detail afterward:

 

 

When you see the three histograms next to each other, does it make more sense? See how the Underexposed histogram is all the way to the left of the histogram window, and is clipped mid-peak? Then note the Well Exposed histogram, and how both edges of the histogram just touch the edges of the histogram window. Finally, see how the Overexposed image's histogram is crammed and clipped on the right. I hope this helps somewhat! Now let's look at some histogram detail.   Image and Histogram Shape Look at the image below. It is well exposed with no serious problems. The entire light range of this particular image fits within the histogram window, which means that it is not too light or too dark, and will take very little or no adjustment to view or print. It contains no more than four or five stops of light range.

Look at the left side of the histogram graph above and see that it does not cram itself against the dark value side. In other words, the dark values are not clipped off on the left. This means that the camera recorded all the dark values in this image, with no loss of darker detail. Then look at the right side of the histogram graph, and note that it is not completely against the right side, although quite close. The image contains all the light values available. Everything in between, such as the blues and grays, are all exposed quite well, with full detail. A histogram does not have to cover the entire window for the exposure to be fine. When there is a very limited range of light, the histogram may be rather narrow.

 

 

 

Let’s look at a couple of examples of histograms on shots that are overexposed or underexposed.

 

Compare these two shots and their corresponding histograms:

 

 

The above shot has a lot of light tones – in fact there are parts of the shot that are quite blown out. As a result on the right hand side of the histogram you can see a sudden rise. While there are quite a few mid tones – everything is skewed right and with the extreme values on the right hand side indicate an over exposed shot.

 

This photo below has a lot of dark tones. This is partly because of the black and navy clothes in the shot – but also because it’s slightly underexposed shot. The resulting histogram is quite different to the first one – the values are skewed to the left hand side.

 

 

 

 

A Perfect Histogram??

 

As with most aspects of photography, beauty is the in eye of the beholder and there’s always a lot of room for personal taste and different ways of expressing yourself as a photographer.

There is no such thing as the ‘perfect’ histogram – different subjects and photographic styles will produce different results. For example taking a silhouette shot might produce a histogram with peaks at both ends of the spectrum and nothing much in the middle of the graph. Taking a shot of someone at the snow will obviously have a histogram with significant peaks on the right hand side…. etc

 

Having said this (and to generalize) – in most cases you’ll probably want a fairly balanced shot with a nice spread of tones. Most well exposed shots tend to peak somewhere in the middle and taper off towards the edges.

 

 

Some Unbalanced Histograms That Work!

 

This low key shot's histogram shows that almost all of the data in the image is down in the lowest areas (darkest) with just a small amount of data showing the bright moon. But since the dark areas aren't right up against the left hand side and the light areas aren't up against the right hand side of the histogram, the subject falls within the dynamic range that can be captured. The detail in the moon is what "makes" this shot.

 

In this "high key" image, below,we see just the opposite. Almost every value seen is toward the right side of the histogram, in the highlight area. That's where the photographer wanted to properly reproduce the brightness found in this snow scene. Yet, since it doesn't bump up against the right hand side of the histogram --none of the highlights are blown out.

 

 

 

Have fun!!! Remember histograms are just a tool to help you take the perfect photograph!! The beauty is in the eye of the beholder!!

 

 

 

 

 

 

 

 

[F, Calla & Luca]

Having finally read this it makes perfectly good sense. Did you write this Diane, because whoever did did a great job explaining a complicated topic. Thanks

[Diane Margetts]

I assembled it from several really good sources so I can only take credit for good plagiarism . . . (smile!!!)

[F, Calla & Luca]

A talent in and of itself. Plus the final product is your own.