Using Histograms to get the Exposure Right

Remember when you used film? You'd take the pictures and deliver the film for processing and wait. And hope.  And do you remember that sinking feeling when you saw that many of the images could have been so much better if the exposure was different?

With film cameras you had the choice of using the built in light meter, using an external light meter or various exposure 'rules' such as 'Sunny 16'. Whichever methods you used, there was still the all too likely possibility that the exposure could have been better.

Digital has changed all that. With the ability to preview the image instantly, it's easy to tell if the image is exposed ok. Yet, if your camera offers a histogram display, then it's possible to go beyond just being 'OK' and get the absolute best exposure.

What is a histogram?

A histogram is a graph that plots the distribution of tones throughout the image. Here's an annotated example:

A histogram shows the Dark tones on the left of the graph (X axis), the light tones on the right (X axis) of the graph and the quantity of tones along the Y axis. In a well exposed image the majority of the tones will be in the middle of the X axis, as is the case in this example.

An under-exposed example

In this example, the image is too dark and the histogram reflects it - the graph shows too many values on the left hand side. Indeed, the highest 'peak' of the histogram is in the darkest tones - a sure sign of an under-exposed image.

An Over-exposed example

The above image has a peak on the right hand side, corresponding to the sky, which is 'bleached'.

A corrected version:

This version was corrected by using a Gray Grad filter to darken the sky. The histogram actually shows that we could have done better - it's now a little under-exposed (the large peak on the left) and we should have exposed it for a bit longer.

Based on these examples, here's a few simple conclusions:

	If there are 'peaks' on the left side of the graph, the image needs more light
	If there are 'peaks' on the right hand side of the graph, the image needs less light
	If there are 'peaks' in the centre of the graph and no 'peaks' on the left and right side then the image is well exposed.

However, that is not the whole story.

Sometimes, you get an image where there are peaks on both sides of the graph. This means that the range of tones is too great to be captured. To solve this there are a number of options:

	Adjust the camera to get the dark tones right and use a gray grad filter to bring in the light tones (as in the example above)
	Take 3 images - one getting the dark tones right and one getting the highlights right and one in the middle and combine them later or use HDR
	Decide what the main subject is and adjust the exposure to get that part right

Option 1 works if it's the sky that is too bright and you own a gray grad filter or two.

Option 2 is best done using HDR techniques.

Option 3 is fine if you are taking, say, a picture of a bird, animal, person or building and it doesn't matter if the rest of the image is too dark/light.

For example, you may be taking a photograph of a white bird, such as a gull. As it's the main subject of the image, you want to expose it so the histogram shows the main light area towards the right of the graph but not at the right edge. Why? Well, the rightmost edge means over exposed. However, if we exposed so that the bird's detail is in the middle of the graph, the bird will be gray and we want a white bird to be, well, white. So, as the great bird photographer, Arthur Morris, put it - 'whites to the right!' when the main subject of the image is white.

The histogram's we displayed above come from Andy's Browser - typically they are much larges and clearer than an 'in camera' histogram. Some in-camera histograms now distinguish between the main colors - whilst others show a simpler graph displaying the overall tones of the image. They are still pretty useful.

So, a good method to use when sizing up a scene is:

Take a test image at the camera's recommended exposure.

View the histogram.

If the graph shows the peaks at the left - add more light

If the graph shows the peaks at the right - reduce the light

If the peaks are only in the middle, then its a good exposure, but you might improve it still by trying to get the peaks a little to more to the right, as long as they don't reach the extreme right.

If the graph shows peaks on both the left and right extremes, then it's not possible to get the whole scene correctly exposed without using filters or resorting to image blending/HDR.

Be aware that the white balance setting of the camera does affect the histogram - try to set the white balance as near to the actual type of light as possible.

The histogram is one of your best friends.