Using the camera’s histogram as you shoot
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.
Caution must be used, however, when reading histograms. Firstly, the histogram only takes up a very small area on screen on DSLRs, so it can be hard to see whether the graph is really at the extremes or not.
In our histogram photography cheat sheet below we’ve illustrated how the back of your camera might look while composing in Live View mode with the histogram enabled.
To see the larger version of this cheat sheet, simply click on the infographic or drag and drop it to your desktop.
Another point to consider is that the histogram is based on a JPEG image of the shot that you are taking.
If you shoot in RAW, the camera will capture more tonal detail (as it is captured in a 12-bit or 14-bit form, rather than the 8-bit limit used by JPEGs); so you may be able to recover highlights that appear blown out in the graph using RAW conversion software.
It’s also worth pointing out that parts of some images should be clipped – simply because they should be shown as pure white or pure black.
Specular highlights (mirror-like reflections from a shiny surface) will blow out whatever you do – and a white backdrop in a studio is meant to look ‘overexposed’, if the background is not to appear dirty and grey.
While you should take care not to set the exposure to avoid unnecessary blown-out highlights, don’t be overly cautious. With digital images, it is always best to expose images so that they are as bright as possible.
The reason for this is that half of the camera’s possible gradations of tone are actually devoted to the brightest 20% of the image. If you underexpose the shot, even slightly, you are throwing away image quality.
As a consequence, the darkest part of the image may end up with more noise and less detail.
Recent DSLRs also 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).
With some subjects, clipped detail in the channels can also help you to ensure you get maximum detail in a particular subject (eg: the petals of a bright red flower). However, for most purposes, the simple black-and-white graph is all you need to avoid exposure pitfalls.
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