ETTR is founded upon the linearity of CCD and CMOS sensors, whereby the electric charge accumulated by each subpixel is proportional to the amount of light it is exposed to (plus electronic noise). Although a camera may have a dynamic range of 5 or more stops, when image data is recorded digitally the highest (brightest) stop uses fully half of the discrete tonal values. This is because a difference of 1 stop represents a doubling or halving of exposure. The next highest stop uses half of the remaining values, the next uses half of what is left and so on, such that the lowest stop uses only a small fraction of the tonal values available. This may result in a loss of tonal detail in the dark areas of a photograph and posterization during post-production. By deliberately exposing to the right and then stopping down afterwards (during processing) the maximum amount of information is retained.
On cameras that have a high noise output from the Analog to Digital Convertor, ISO can be altered to Expose To The Right also. In this situation where you cannot use the shutter and/or aperture to ETTR, and the sensor has not been saturated, you can use ISO to ETTR in the same manner as using shutter and/or aperture. Using ISO in this manner will boost the signal from the sensor to the saturation level of the Analog to Digital Convertor, increasing the Signal to Noise ratio of the output from the Analog to Digital Convertor.
This technique is only relevant for use when shooting in a raw image format then processing in a raw converter before exporting the file to a raster graphics editor. If the technique is used with JPEG files (the default on most consumer cameras) it will not work as intended and may result in overexposed pictures for low-contrast scenes and underexposure for high-contrast scenes.
^Martinec, Emil (2008). "Noise, Dynamic Range and Bit Depth in Digital SLRs". Retrieved 4 February 2014. "The proper reason to expose to the right comes from figure 12 on page 2, showing the rise in signal-to-noise ratio with increasing exposure. By increasing the number of photons captured, the S/N ratio improves, and the image quality improves directly in proportion to that improved S/N ratio."