Exposing to the right

Histogram of a low-contrast scene, normally exposed.

Histogram of a low-contrast scene, exposed to the right (+1 EV).

In digital photography, exposing to the right (ETTR) is the technique of increasing the exposure of an image in order to collect the maximum amount of light and thus get the optimum performance out of the digital image sensor. The name comes from the fact that the resulting image histogram should be placed close to the right of its display. Advantages include greater tonal range in dark areas, greater signal-to-noise ratio (SNR), fuller use of the colour gamut and greater latitude during post-production. ETTR images appear to be overexposed when taken and must be correctly processed (normalized) to produce a photograph as envisaged, therefore care must be taken to avoid clipping within any colour channel, other than acceptable areas such as specular highlights.

The principle is also applied in film photography in order to maximize the negative's latitude and density and achieve richer blacks when the image is printed slightly down.

Background

ETTR is founded upon the linearity of CCD and CMOS sensors, whereby the electric charge accumulated by each subpixel is directly 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 can result in a loss of tonal detail in the dark areas of a photograph and posterization during post-production. The lowest stop also has a worse SNR than higher stops. By deliberately exposing to the right and then stopping down afterwards (during processing) the maximum amount of information is retained.

The technique was first described in 2003 by Michael Reichmann on his website, after purportedly having a discussion with software engineer Thomas Knoll, the original author of Adobe Photoshop and developer of the Camera Raw plug-in.

Limitations

This technique is only intended 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.

Exposing to the right by increasing the ISO setting (in digital camera systems) will not work as intended and may result in increased noise.

External links

• http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html#ETTR - Noise, Dynamic Range and Bit Depth in Digital SLRs
• http://www.luminous-landscape.com/tutorials/restore-clipped.shtml - Restore Those Clipped Channels
• http://www.luminous-landscape.com/forum/index.php?topic=9510 - Revisiting the "expose to the right" dogma
• http://chromasoft.blogspot.com/2009/09/why-expose-to-right-is-just-plain-wrong.html - Why "Expose to the Right" is just plain wrong
• http://www.guillermoluijk.com/article/ettr3/index.htm - Explanation of ETTR in Spanish