Exposing to the Right

Exposing to the right (often refferred to as ETTR) is a technique that seems to polarise opinions across the internet so you can find plenty of examples of people supporting its use and an equal number of people claiming it adds no value.  The principles of the technique however do hold value and are valid to consider when out taking images.  As it is a technique that I regularly employ when photographing landscapes I wanted to share the reasoning behind it, and show an example of the benefit it can bring.

The term ‘expose to the right’ refers to the histogram associated with an image.  Typically, for a shot to be well exposed, we are taught to aim for an even spread of tones across the histogram, peaking in the middle, and tapering off at the edges.  When ‘exposing to the right’, the idea is to push the peak of the histogram as far to the right hand side as possible, i.e. overexpose the image, without clipping any highlights.  The resulting file, when processed back to the correct exposure, will contain more tonal information and less noise in the shadow areas, maximising your image quality.

Left: A histogram showing a 'correct' exposure. Right: An 'exposed to the right' histogram

Let’s consider the CCD or CMOS sensors found in most digital cameras.  Typical DSLR sensors can capture seven stops of dynamic range and produce 12-bit raw image files, capable of recording 4096 tonal levels in each red/green/blue channel.  The ability to record such a large number of tones should guarantee smooth transitions between the tones within the resulting image, however it is not quite that simple.

Whilst you might think that each of the seven stops in the range of the sensor record an even number of tones throughout the dynamic range, you would be mistaken.  F-stops are logarithmic in nature meaning that each stop records half of the light of the previous one.  Practically, this means that the brightest stop records half of the possible number of tones, i.e. 2048, the second stop records half again, i.e. 1024, and so on until the seventh stop that records only 32 tonal levels.  Therefore, if you underexpose an image and correct the exposure during in post processing, the tonal transitions in the darker areas will not be as smooth, and the risk of degrading your image quality is much higher.  If you overexpose your image, by pushing the histogram to the right, you will capture much more tonal information that results in much better image quality when correcting the exposure in post processing.

The diagram below tries to illustrate the distribution of tones for each stop of the dynamic range of the sensor.  The top image shows the seven different stops capturing different portions the dynamic range from the darkest through to the brightest tones, however the bottom diagram shows those stops but sizes them relative to the number of tonal levels that each stop captures.  As you can see, number of tonal levels captured by the brighter stops is significant compared to the stops at the lower end of the dynamic range.

A tangible way of demonstrating the difference in the amount of tonal information recorded is to take two images of the same scene, one underexposed, one overexposed and compare the file sizes: the overexposed raw file will be larger that the underexposed shot as it contains more data.

Let’s look at an example.  The image below shows two unprocessed shots taken within seconds of one another, with their associated histograms.  The shot on the left is underexposed and the shot on the right is exposed so that the histogram is pushed up to the right hand side, as far as practically possible wihout losing any highlight detail.

Left: Underexposed image. Right: Exposed to push the histogram to the right

During processing, the exposure of each shot can be adjusted to give what seem to be two identical images.

Left: Underexposed image. Right: 'Exposed to the right' image. Both undergone exposure correction during post processing

However, when you look in detail at a 100% crop of an area of each image, you can see a huge difference in the quality of the final image.  The shot that was underexposed (i.e. exposed to the left) shows much less smooth transitions between tones and much more noise in the darker areas than the image that was exposed to the right.

Left: Underexposed image. Right: 'Exposed ot the right' image. 100% crops to demonstrate diference in image quality.

Images that have been exposed to the right will need some additional post processing to correct the exposure, but as you can see, a bit of extra thought when determining your exposure and some extra steps to correct it during post processing can result in image files with smoother tonal transitions and reduced image noise.

It is not a technique that is universally applicable to all types of photography, as there is a risk of clipping highlights if care is not taken when exposing your image.  Exposing to the right is most suited to when photographing in a controlled environment, for example, when shooting landscapes, using graduated filters to ensure that all highlights are contained within the dynamic range of the sensor.  The last thing you want to do is to lose highlight detail when trying to maximise your image quality.

So give it a go, take two images at different exposures (one exposed normally, one exposed to the right) and see if you can see a difference.  Understanding the performance of your individual sensor in such a way is a step further to knowing how to get the most out of your camera.