ETTR or Exposing to the Right is an advanced way of looking at exposure of digital photographs.
ETTR means taking a photograph in such a way that the histogram shifts towards the right side or towards the White Point.
This essentially relates to the data capturing technique of the digital camera sensors. Towards the left side of the histogram or in the darker areas of the image there are very few photons that get recorded by the photo diodes or photo sites on the sensor. As a consequence, the available information to create the complete picture is much less and the cameras processor, under such circumstances, is more prone to generating noise in the resultant image.

Before getting into more technical discussions, let’s establish some obvious facts about exposure.

Exposure is the combined effect of ISO, shutter speed and aperture settings at the time of taking the photograph.
In other words it is the combined effect of the photo-sensitivity, duration of light and amount of light falling on the camera’s sensor. Depending on the availability of light as well as light to shadow ratio, the distribution of tonal variations would change.

The maximum possibility of capturing of tonal ranges varies from the Black Point to the White Point on the luminosity axis. The black and white points are represented by the extreme left and right points on the horizontal axis of the histogram.  Between these two points lie all the shades of luminosity.

The complete breakdown of the luminosity ranges with darkroom procedure methods was done by Ansel Adams. He called this “the Zone System”.
If one understands the zone system, the understanding of overall exposure and the roles that different illuminance values play becomes very clear. Although the zone system can at times appeared to be little complicated, one can always look at it as a more simplified pattern, especially in the context of digital photography. The original zone system was designed and empirically calibrated for various darkroom procedures of film developing and printing. In that regard it can appear little complicated. However the digital darkroom procedure is are significantly different from the film treatment methods.

Ansel Adams created a zone system consisting of 11 luminosity zones. Zone 0 was the black zone while zone 10 was the white zone. These zones were not random divisions but they correspond to the actual exposure values in photography.
Ansel Adams zone system may not be applicable directly to the exposure systems of digital photography. However, it provides a very clear understanding in terms of luminosity distribution across the frame.

There are multiple arguments and views about application of Ansel Adams zone system to digital photography. One of the simplest and logical way is to look at creating parallel zone system for digital photography. And this system would perhaps consist of only five zones.
These 5 zones can be mapped to the Blacks, Shadows, Mid Tones, Highlights and Whites.
These are also the same zones that are available as editing tools in Adobe’s Camera Raw as well as Light Room software.

Out of the 11 zones defined by Ansel Adams, zone number 0 and 10 which represent pure black and pure white respectively, are actually inconsequential in digital photography (zone 0 having no data and zone 10 having data overflow).
Thus, in digital technology, one is primarily dealing with zones between 1 and 9.
Out of these, both the extremes are likely places for blackouts or washouts and do not hold a significant amount of details of the image.
Most parts of the image details are contained in the zones from 2 to 8. Ansel Adams called this range as “the Texture Range”.
It is this range that has the maximum separation for different wavelengths thereby giving more details in the photograph.

Having understood the structure of Ansel Adams zone system, let us try and implemented to the practical digital photography.

Let us consider that you have a photograph that has the zone values varying from 2 or 9. When we try to expose a photograph with such large dynamic range, we tend to keep more focus on the shadow areas trying to bring out details there in. In an attempt to do so the usual side effect is to have the highlights totally blown off. On the other hand if we try to prevent the highlights from getting blown off, we end up having the shadow areas very dark and without much of details.

Understanding the ETTR method allows us to have greater retention of details in both highlight and shadow areas.

When you use spot metering with your camera, it tries to bring the luminance of area metered for close to 18% grey luminosity value.
18% grey correspond to zone number 5 on Ansel Adams scale.

If you spot meter in the darker area, say zone 3, then the overall exposure will get elevated in such way that zone 3 shifts to zone 5. The original zone 6 will now become 8 and start showing blown-off effects, while original 7 will be a certain washout! (Refer below left side image)
As a cardinal principle of photography, it is acceptable to have more shadows in the capture, but the blown off areas should not exceed 5% to 7% of the area.

In the earlier example, all the areas in zones 6 and above will start having over exposure and the area will be too large to be acceptable.
Spot metering in the highlight area, on the other hand will push the scale downward, thereby avoiding burning in the photograph. (image above, right)