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How to use the histogram to get a better exposure? Options
photoling
Posted: Sunday, March 13, 2016 10:19:56 AM

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One of the first things you probably want to know after taking a photo is whether or not you got your intended exposure.

You might be tempted to look at the preview on your LCD screen, but when you’re shooting photos outdoors in the sun, those previews can be very deceiving, making your photos look much brighter than they actually are.

So, how do you know if you got the right exposure?  The better way is to use the histogram. Knowing how to read your camera’s histogram is the most important thing you can do to ensure a good exposure.

What is the histogram?

The histogram offers a graphical display of exposure. It basically just shows the distribution of light and dark pixels in your image.

Here’s an example, with each axis labeled:

histogram-detail

The histogram is a graph where the horizontal axis indicates different brightness levels (a 256-step scale), with the darkest tones at the left and the lightest tones on the right. The vertical axis shows how many pixels there are in the image at any given brightness level. For example, if you take a short exposure with your lens cap on then the image will be essentially black, so you’ll get a single spike at the extreme left-hand edge of the histogram display. Take a long exposure of a brightly lit sheet of white paper, and you’ll get a single spike at the right-hand edge of the histogram. So, in this example, the image mostly has dark to midtone pixels.

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Posted: Sunday, March 13, 2016 10:19:56 AM
photoling
Posted: Sunday, March 13, 2016 10:49:38 AM

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How to use the histogram

A good image often, but not always, has a histogram spread all over. Contrary to popular opinion, there is no such thing as an ‘ideal’ histogram. Some subjects will give shots that simply have more brighter tones than others – and ultimately no two histograms will be identical.

It’s the overall shape of the graph that is your secret weapon for assessing the shot’s exposure and contrast.

To get the best tonal range, and to avoid problems with underexposed shadows or overexposed highlights, the histogram should be vaguely bell-shaped. That is, the graph should drop down to baseline towards both left-hand and right-hand extremes.

What you should try to avoid is an image with the graph stacked to the extreme right or left – as this suggests that detail has been lost or ‘clipped’ in the highlights or shadows.

For scenes that are fairly low in contrast, a correct exposure will have quite a wide peak with gently sloping sides, and it should be roughly in the middle of the horizontal axis. If the peak is towards the left, it means that the image is under-exposed and too dark. If the peak is towards the right, the image is over-exposed and too bright. You can therefore review your shots on your EOS camera’s LCD using the histogram view, make any required changes to exposure compensation, then re-shoot.

Things get a bit trickier in high-contrast scenes. Here you’ll tend to get multiple peaks representing dark, midtone and bright areas in the scene. The thing to consider is what the most important part of the scene is. If you want to accentuate detail in darker areas, apply over-exposure so that the left-hand peaks move over towards the centre.

You’ll notice that the right-hand peaks corresponding to bright areas may move to the extreme right-hand edge, meaning that you’ve got ‘blown highlights’, or that they’ve ‘washed out’ to pure white. This is also indicated by flashing areas on the thumbnail image displayed next to the histogram. However, in a backlit portrait, for example, it’s much more important to have a good exposure of the face, even if it means the background washes out.

Conversely, if you want to retain detail in the highlights of a high-contrast scene, it’s vital to ensure that the right-hand peaks are kept slightly in from the histogram’s right-hand edge, even if you end up with left-hand peaks being pushed over towards the left edge of the display. An example of this is where you want to keep rich colour and detail in the bright skies of a landscape, even if it means that underlying areas of land look quite dark.

photoling
Posted: Sunday, March 13, 2016 11:08:06 AM

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There are actually two histogram display options – brightness and RGB – but the brightness histogram is generally the most useful. Cameras can’t reproduce an infinite tonal range, or range of brightness levels, so you need to alter the exposure settings depending on the brightness of the scene. It’s a bit like the way the pupils in your eyes contract when you walk out of a dark room and into the sunshine.

Recent DSLRs give the option to show a colour histogram, in addition to the black-and-white luminosity version. This RGB histogram shows three separate graphs, corresponding to the red, green and blue channels that the picture is made up of.

RGB graphs can be useful for a number of reasons. If there is a marked difference in the three graphs, it can give an indication of a white balance problem (though this may simply show that one particular colour dominates the composition).

photoling
Posted: Monday, March 14, 2016 11:34:46 AM

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In digital imaging, overexposure is difficult or impossible to correct for at the editing stage – so with high-contrast subjects it is better to have an image that is stacked to the left, than one that it squeezed up tight to the right.

As it’s possible to recover underexposed areas using image manipulation software.

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