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Tips on histograms

A helpful tool for the photographer to adjust exposure & contrast.


A histogram in photography is useful in analyzing exposure in your images.
A histogram in photography is useful in analyzing exposure in your images.

A histogram is a bar chart, a graphical display of data using bars of different heights. The bars represent all of the tones in a digital image. You can use a histogram to understand and adjust exposure, and especially to assess whether image detail is missing due to blown-out highlights or black shadow areas.

You may be wondering where you will come across a histogram. Well, many if not most digital cameras have the ability to show the photographer a histogram of an image he or she has taken. (Check your camera's manual to find out how to display a histogram of an image you have just taken.) And most image editing applications can display a histogram for an image you have already taken and downloaded. (Click on the Help section of your image-editing application to find out how to show a histogram for an image that you are editing.)

Are they important, you may ask? Yes. In fact, many photographers consider a histogram to be one of the most useful tools in their arsenal for the creation of quality imagery. They are a huge help in properly exposing your pictures as you take them. And later, while you're in front of your computer, they are incredibly useful in assessing the quality of an image and in enhancing the image by improving brightness and contrast to make it look its best.

The diagram below shows a histogram for a black and white image. (We discuss color image histograms further below.)

Each vertical bar in a histogram represents one shade of gray and the amount of it that appears in an image. The bars on the left show dark areas, lightening to the right until they reach pure white.
Each vertical bar in a histogram represents one shade of gray and the amount of it that appears in an image. The bars on the left show dark areas, lightening to the right until they reach pure white.

Grayscale (black & white) images contain as many as 256 shades of gray. Each vertical bar in a histogram represents one of those shades of gray and the amount of it that appears in an image, ranging from a solid bar on the far left representing black (0) to a solid bar on the far right representing white (255).

  • The histogram for a properly exposed image will show a lot of darker tones with a small amount of solid black tones (indicating deep shadow where detail is not visible) and a small amount of white tones (indicating over-exposed areas where detail is non-existent).
  • An underexposed image will probably show no white bar in its histogram.
  • An overexposed image’s histogram will look like a reverse or mirror image of an underexposed image, with no black bar on the left.
  • The histogram for a flat image (one that lacks contrast) will display bars concentrated in the middle and no black or white tones measured.
  • Think of a histogram as a graph that shows how many of the light levels in a scene (the darkest to the brightest) have been captured by your camera. The vertical bars represent the size of the area in an image for each of these zones.

    The histogram for a very dark image has most of its bars grouped towards the left side of the chart. A very bright image with few dark areas has most of its bars grouped towards the right side of the chart. Perhaps the easiest way to consider the information a histogram tells you is to remember "Deep shadows on the left; bright highlights on the right; mid-tones in the middle."

    When an image’s histogram contains bars that are mainly located very close to one side or the other, the camera's image sensor was unable to properly record and represent the scene as it should be viewed. Too close to 0 (dark black) and the image will be too dark to reveal any detail. Too close to 255 (pure white) and there will only be light and white tones.

    Bars too close to the left indicate that the image is too dark to reveal detail. Too close to the right indicate too many light tones.
    Bars too close to the left indicate that the image is too dark to reveal detail. Too close to the right indicate too many light tones.

    This tonal information can often be adjusted in an image-editing application like Adobe PhotoShop to render an image so that it appears to have been more properly exposed. But, you should not put too much reliance on PhotoShop, as good as it is, or any other program to correct your image's exposure errors. Why? Because sometimes, often in fact, they cannot salvage an image that just doesn't contain sufficient detail where it should be in the first place - the original photograph. You are always better off to properly expose a picture when taking it. Your camera's histogram can help you to judge whether you are doing that.

    You can use your camera’s histogram capability in real time, when you are taking the original photographs, to assess the contrast and exposure characteristics of an image you have just taken, permitting you to change exposure settings to re-shoot the scene to achieve a more acceptable result.

    Once you get used to using a recently-captured image's histogram on your camera to evaluate exposure as you are taking pictures, you will probably use histograms all the time when shooting to improve exposure in the next frame you shoot. It takes only four or five seconds to check a histogram on your camera, and those brief moments can save you a lot more time later on when trying to improve that important shot on your computer.

    As a rule of thumb, your objective is generally to capture an entire tonal range in the images you are shooting, and a histogram can help you to achieve that.

    A color histogram showing red, green and blue channels in an image.
    A color histogram showing red, green and blue channels in an image.

    COLOR IMAGE HISTOGRAMS

    The histogram for a color image is assessed in a similar manner, with the difference being that a color histogram graphically shows the distribution of colors in a digital image, with its bars representing the number of individual colored pixels present.

    A color image’s histogram is composed of separate red, green and blue (RGB) color channels shown by RGB bars in the chart. Color histograms are actually composed of three individual histograms, one each for red, green and blue, useful in identifying and correcting color-related problems in an image.

    A color histogram separated into its red, green and blue channels in an image-editing application.
    A color histogram separated into its red, green and blue channels in an image-editing application.

    A PRACTICAL TIP TO IMPROVE IMAGE QUALITY

    Your camera’s sharpening or contrast setting influences tonal range.
    You can increase the number of tones in your images by changing this setting.

    Many cameras give you the ability to adjust sharpening and contrast as you take pictures.

    If you have such a camera, and you are photographing a high contrast scene, changing your sharpening (or contrast) settings to "low" will give your pictures more tonal range.

    A "high" sharpening or contrast setting creates greater contrast between an image’s individual pixels, often resulting in reduced tonal range.

    More tonal range means that your digital image file will have more information about the picture and its exposure details. This is a good thing, giving you more to manipulate to improve your image.

     
    Further information...
    Exposure
    Basic metering with your camera's meter
    Bracketing exposure
    Brightness of Light
    Correcting an underexposed digital picture
    Editing digital images
    Related topics...

    Image resolution

    Editing digital images

    Gray card

    Image resolution

    Light and its color

    Overexposure & underexposure