Sunday, June 8, 2014

Exposing to the Right

Exposing to the Right

"The term ‘expose to the right’ refers to the histogram associated with an image.  Typically, for a shot to be well exposed, we are taught to aim for an even spread of tones across the histogram, peaking in the middle, and tapering off at the edges.  When ‘exposing to the right’, the idea is to push the peak of the histogram as far to the right hand side as possible, i.e. overexpose the image, without clipping any highlights.  The resulting file, when processed back to the correct exposure, will contain more tonal information and less noise in the shadow areas, maximizing your image quality."

Expose to the right - histograms
        Left: A histogram showing a 'correct' exposure. 
Right: An 'exposed to the right' histogramhistogram showing a 'correct' exposure. Right: An 'exposed to the right' histogram
"The diagram below tries to illustrate the distribution of tones for each stop of the dynamic range of the sensor.  The top image shows the seven different stops capturing different portions the dynamic range from the darkest through to the brightest tones, however the bottom diagram shows those stops but sizes them relative to the number of tonal levels that each stop captures.  As you can see, number of tonal levels captured by the brighter stops is significant compared to the stops at the lower end of the dynamic range."
Expose to the right - tonal distribution
Additional resources:

Incomplete use of a camera's dynamic range reduces the number of recorded tones, and this problem is compounded by the fact that cameras capture disproportionately fewer dark tones compared to how we see with our eyes:
(A) Scene captured using the full tonal range:

(B) Scene captured using only darker tones:under exposure
. . . same scene after being brightened to match (A) above:
under exposure with exposure compensation
Note: the number of tones has been reduced for visibility, but the same trend applies.
The actual number and distribution of tones depends on the bit depth and gamma.
See the tutorial on gamma correction and digital tones for more on this topic.


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