Histogram

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The histogram is a graphical representation of the brightness levels in an image and every digital image on your computer has one.

As a matter of fact most quality digital cameras have the ability to display this graphical representation of image brightness for each image and every one of the quality digital editing programs (Photoshop, Photoshop Elements and Paint Shop Pro) prominently display the image histogram.

Understanding what it is telling you may be a great help in your projects. The information that follows came from different web sites and my own observations.

Numbers, Numbers, Numbers

The shape of a balanced histogram is almost identical to the normal curve which is used extensively in pretty much every field of study. The normal curve is used by psychology, social sciences, advertisers, retailers and the list goes on and on.

The Ends

The left end is the Shadow area and the right end is the Highlight area.

The area roughly one-third from the left to one-third from the right are the mid-tones. In this image the majority of the pixels are in the mid-tone area with a nice representation of both shadows and highlights so it is a nicely balanced image.

Tonal Range

Ideally the best image will have pixels at each level from 0 to 255 in a shape similar to this one. It really is rare to have a histogram like this and I believe out of the 1000's of images on my computer this was the only one with a nice balanced shape!

The higher each vertical band - the more pixels are present at that particular level of brightness.

At brightness level 162 on this histogram (the highest point) there are 162,643 pixels and at the left end (level 0) there are a mere 11,248 pixels. The entire image contains 23,970,816 pixels in total.

The total number of pixels and the amount of pixels at each brightness level are not important - just interesting unless of course you are some kind of math geek. What is important is the shape and where the end points are located.

Middle Gray

The metering of every digital camera light meter and every hand held light meter is standardized to set the cameras f-stop and speed based on an object that is middle gray at approximately 18% luminance. If the camera is pointed at a very dark subject the meter will assume that is 18% gray and make the settings accordingly - this is the main reason for both under exposures and over exposures so be careful when you are metering.

It is for this reason that high end camera stores sell 18% neutral gray cards to help photographers get the metering correct right in the camera.

On a digital photo histogram (Photoshop, for instance) the middle gray point falls at luminance level 128 - half way between 0 (black) and 255 (white) and it's labeled Median. Take a look at median on the histo at the top of the page and the medians on the ones that follow.

When working with Levels, middle gray is labeled as 1.0 and if the image was exposed incorrectly the mid-point slider can make up the difference.

When working with curves the middle of the curve line can be pulled up or down to change the middle gray point.
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Different Histograms

There really is no such thing as a bad histogram because each image has its own purpose. Here are some examples and some observations.

		The Ballagio A night shot and the brightness levels stretch all the way from 0 to 255. The pixels that run up the on the Shadow side are clipped - they are completely black with no detail at all.
		City Center This image is really flat and way too dark. The brightness levels only cover 201 levels leaving 54 with no pixels. This is a job for Levels or Curves.
		Tabitha and Libby My two white girls playing in the snow does not make for a very wide tonal range, does it? This image is lacking contrast. I begins at level 42 and stretches to level 191 so 106 levels of brightness are missing. Levels or Curves will fix it up.
		Waterdown This image is lacking in the Highlight area - 26 levels of brightness are missing. A simple levels adjustment fixes this one up nicely.
		Maple Syrup This is a very old image that was scanned. It reacted very well to a levels adjustment. It lacks contrast from the shadow end and has some clipping in the highlights. This one is missing the first 33 levels of brightness.
		Low Key Image This was shot in a casino in Las Vegas with a flash. The bright parts of the image fooled my flash so it is all clumped up in the shadow area but effective none the less. A curve adjustment will make this one more interesting. This would be classified as a low key image.
		High Key Image All of the brightness levels of this image are in the high mid-tones and the highlight areas, other than the eyes. This is how it is supposed to look and it took a lot of dodging and burning to get it that way. Doing any further adjustments would really be pointless, don't you think?

Adjusted Histograms

When either a curves or levels adjustment is done on an image the curve changes - perhaps the adjustment caused it to spread from 0 to 255. It also has these holes or blank spaces as well.

Waterdown Adjusted Histogram

There is nothing wrong with the adjustment - this is normal. If the original image covered 220 levels of brightness and covers the full 256 after the adjustment then something has to give. No more pixels were added so some levels, well, they go away and that is what those blank lines are - the missing levels of brightness.

The good thing is that you will never notice where the missing levels of brightness are located.

The End For Now

If you listen to the professional photographers they will tell you the best plan is to get the image correct in the camera and no real adjustments will be required later.

Seeing as how this is a site about digital imaging .... keep on editing!

Measuring The
Tonal Range

This is relatively easy to do. First, make sure the Histogram is in Expanded View.

Place the cursor inside the histogram graph at the point where the graph is just starting , left click and drag to the other side.

In Expanded View next to Level it will show you the starting point and the ending point.

Normal Curve

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