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Colourful Muizenberg beach shows a good example of a histogram that has contrast and colour.

Histograms are an excellent way to check your exposure, but you need to know how to read them for your camera. It may seem complicated to start with but it is actually quite simple when you get the hang of it. The most difficult part is actually seeing the LCD screen in a the outdoor sunlight so you can read it.

What is a Photographic Histogram?

A histogram is a graph that depicts data but in the case of photography the tonality of an image is shown on the horizontal axis and the number of pixels present in the image on the vertical axis. The horizontal axis also indicates the levels of contrast an image displays.

This is what a Histogram looks like.Hist AC Muizenburg

 

The top histogram is a composite of the three colour channels and the luminosity histogram.

The three below it are named for their respective colour channels.

These represent the various tones that make up the image.

The height of the graph is a depiction of how many pixels there are in the image of a tone at that particular point and therefore shows how much of the image is found at that point.

Also shown in the horizontal graph space are five sections that that represent Very Dark, Dark, Mid-tone, Light and Very Light. These go from left to right and are covered from 0 to 255 of individual brightness points. This makes 256 points. The Dark can have twice the information that the Very Dark area contains and the Mid-tone twice what the Dark contains – and so on as you move to the right of each section. (See diagram below) This is why people often say expose to the right!

A spread across the five sections shows lots of contrast, but if you overflow to the left the image, parts are underexposed and to the right parts are overexposed.

Each photograph is going to have its own unique histogram and no histogram can be said to be correct or ideal as it depends on the tones, contrast, brightness and the Dynamic Range of your camera as to what it will look like.

This is the histogram for the header photograph and you can see that there is a small overflow on the left in the blue channel and on the right in the red channel, but not enough to adversely affect the exposure although you can just begin detect some loss of detail in the red channel.

A Histogram Demo 1

Reading a Histogram.

An evenly spaced histogram that does not overflow the horizontal edges, no matter how high it is will always give a good exposure. You want to aim for a histogram that keeps within these boundaries but usually fills the horizontal space.  This does not mean that histograms that don’t do this can’t show good exposures. If the image is made up of very dark tones and colours it will overflow to the left and if the image has a large white background it will overflow to the right. It is where there is an even tonality and the histogram overflows to the left or right that the exposure must be adjusted.

Different parts of an image may have different histograms.

The Channel Histograms.

These should also be checked as a blown channel can mean loss of detail, especially in the red channel.

If you want to learn more, most of my courses have a Histogram component in them. Give me a ring or email me and we can tailor a workshop to suit your needs.

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