Training & Tutorials

HISTOGRAMS - A Basic Introduction

This brief tutorial explains the basic concepts behind histograms that can be displayed in image editing programs.

The LEVELS and CURVES tutorials (available via the on-line tutorials page of this site) demonstrate how these basic principles can be applied when you are making Levels and Curves adjustments in Photoshop or similar progams.

This tutorial was prepared by Roy Killen and is copyright. It is part of the supplementary training materials for members of the Belmont 16 Ft Photographic Club and is not intended as a complete, stand-alone instruction package.

Comments and suggestions on this tutorial are welcome - they should be emailed to Roy Killen.

Consider the simple image on the right and imagine it is a photograph.

It has pixels of just two tones - black and white. The percentage of pixels of each tone is as follows:

sketch-1-table

If we draw a histogram to represent the distribution of pixels in this image it will look like this:

If we change the image by adding a third tone (the dark grey spot on his shirt) this will reduce the number of white pixels - giving the following distribution:

The histogram to represent the new distribution of pixels in this modified image will look like this:

If we add a fourth tone (light grey hair and shorts) this will reduce the number of white pixels further - giving the following distribution:

The histogram to represent the distribution of pixels in this modified image will look like this:

If we add two more tones (mid-grey for the foreground and pale grey for the background) this will reduce the number of white pixels further - giving the following distribution:

The histogram to represent the distribution of pixels in this modified image will look like this:

You should notice several things about the above histograms:

As we add more tones (different shades of grey) we get more "bars" in the histogram.
No matter what tones we add, the "limits" of the histogram cannot extend beyond black (extreme left) and white (extreme right).
The shape of the histogram gives you immediate information about the relative number of pixels at each tonal level. For example, if you look at the last histogram you can see immediately that there are more "pale grey" pixels than "white" pixels.

If we have a monochrome image that contains some pixels of every shade of grey from black to white the histogram will appear to be a continuous curve rather than a series of bars (as in the above examples). The reason for this is that the horizontal axis of the histogram in most image editors is divided into 256 levels (i.e. 256 different shades of grey) which are so narrow that they blend together to produce what looks like a continuous curve. This is illustrated in the following example:

The extreme left of the horizontal axis represents a level of "0" - total black.

The extreme right of the horizontal axis represents a level of 255 - total white.

The height of the histogram at each point on the horizontal axis represents the relative number of pixels in the image at that tonal value.

DISPLAYING HISTOGRAMS IN PHOTOSHOP

When you open an image in Photoshop (or a similar image editing program) you can display a histogram for that image in several different ways. The simplest way is to select Window>Histogram from the menu:

This will open the Histogram palette and give you the option of selecting from several different views (such as the Expanded View with Statistics shown below).

More detailed information about the Histogram palette in Photoshop CS3/CS4 will be provided in a later tutorial.

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This page last updated 12the May 2009 (RK)