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Exposing to the right

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Exposing to the right

Histogram of a low-contrast scene, normally exposed.
Histogram of a low-contrast scene, normally exposed.

Histogram of a low-contrast scene, exposed to the right.
Histogram of a low-contrast scene, exposed to the right.

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),[1] 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 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.[2]

The technique was 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.[3]

Limitations

There are limitations to exposing to the right. The most obvious drawback is the risk of blowing highlights without realizing it, as live histograms are almost always based on JPEG processing, which doesn't accurately reflect the raw data. Images exposed to the right will need post-processing to correct the exposure,[4] so there is also some question about the benefits of using the technique when shooting JPEGs.

References

  1. ^ 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. 
  2. ^ 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).
  3. ^ http://www.luminous-landscape.com/tutorials/expose-right.shtml - Expose (to the) Right
  4. ^ Hook, Elliot, writing at http://digital-photography-school.com/exposing-to-the-right/

External links

  • http://www.luminous-landscape.com/tutorials/optimizing_exposure.shtml = Optimizing digital exposures using ETTR
  • 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
  • https://howgreenisyourgarden.wordpress.com/tag/expose-right/
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