How to Photograph the Milky Way in Really Heavy Light Pollution Using ETTR (Expose to the Right)
Expose to the Right: Correct Exposure in Single-Shot HDRs - From the HDR Photography Workshop Series
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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 derives from the resulting image histogram which, according to this technique, 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.
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.
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.
||This section possibly contains original research. (April 2014)|
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.
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