Reading THe exposuRe HisTogRam

It can be very difficult to gauge exposures in a digital camera’s monitor screen or electronic viewfinder, and a much more reliable guide is provided by the histogram display. Some cameras provide a real-time histogram display with the live view image, but arguably more useful is the display provided in image review after capture. You can read the histogram to see what corrections might be useful.

The histogram is a graphical representation of image brightness levels from black (i.e. the darkest pixels) at the left through to white (the brightest pixels) on the right. This corresponds to brightness values of 0 (black) to 255 (white). This scale is derived from the binary code that represents the building blocks of a digital image, and relates to bit depth. In other words, how many bits – pieces of computer data – are used to represent each pixel in an image.

Digital cameras output images at 8-bits per RGB channel (although they may process the data internally at a higher bit depth). So, 2 to the power of 8 = 2x2x2x2x2x2x2x2 = 256 levels or, in digital terms, 0 to 255 brightness levels. The histogram shows the number of pixels recorded at each brightness level across this range.

So a histogram showing a lot of ‘spikes’ to the left side has a predominance of darker pixels and so represents an underexposed image. Conversely, a histogram with a lot of spikes towards the right has a predominance of brighter pixels and so represents an overexposed image. It’s been said that you should aim for the shape of a mountain in your histogram – i.e. the spikes are predominantly in the middle – as this represents a ‘correct’ exposure. This is technically true, but doesn’t take into account the possibility that you may want an image that is either underexposed or overexposed. Consequently, use the histogram as a starting point for making corrections or achieving creative outcomes.