Exposing to the right
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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 converter, 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 converter, increasing the Signal to Noise ratio of the output from the analog-to-digital converter.
There is a great deal of controversy about the benefits of exposing to the right. The most obvious drawback is the risk of blowing highlights without realizing it. In addition, Sham Bhangal, in his blog, "Howgreenisyourgarden" makes a case, complete with exhibits, for the idea that you lose accurate color reproduction when you overexpose then correct the exposure in post-processing, with colors becoming washed out to some degree. The reason, he explains, is "because the red, green and blue components of your image do not change at the same rate as you overexpose and then underexpose the image." Despite this drawback, he agrees that shooting to the right can be worth it when shooting in low light at high ISOs that are very noisy. Images exposed to the right will need post-processing to correct the exposure, so there is also some question about the benefits of using the technique when shooting JPEGs.
A further complication is that the LCD review, histogram, and over-exposure 'blinkies' are almost always based on JPEG processing, which have a narrower latitude to that of the raw file and raw values in the sensor. With Canon DSLR's, Magic Lantern has implemented raw histograms, review, zebras, and blinkies. Other cameras can attempt to use UniWhiteBalance as a work-around, usually resulting in a strong green cast on the LCD review and the actual image. If the camera has the equivalent of 'Picture Styles', the contrast and saturation can be reduced to minimum.
- 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."
- Fraser, Bruce, "Raw Capture, Linear Gamma, and Exposure," Adobe white paper posted at http://www.adobe.com/digitalimag/pdfs/linear_gamma.pdf?PID=3662453 and adapted from his book "Real World Camera Raw" (2003).
- http://www.luminous-landscape.com/tutorials/expose-right.shtml - Expose (to the) Right
- Bhangal, Sham, "Shooting to the right (and why it can be a bad idea)" published at https://howgreenisyourgarden.wordpress.com/2011/01/10/shooting-to-the-right-and-why-it-can-be-a-bad-idea/
- Hook, Elliot, writing at http://digital-photography-school.com/exposing-to-the-right/
- "Unitary White Balance".
- 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.sensorgen.info/ - Sensor details for various cameras
- http://photography-on-the.net/forum/showthread.php?p=8534003#post8534003 - HAMSTTR, using shutter/aperture/ISO to ETTR