Raw Tuesday: how to assess and adjust the exposure of your raw files

Raw Tuesday: how to assess and adjust the exposure of your raw files

The histogram and colour


Editing raw files: the histogram and colour

You can also keep an eye on the separate colour channels. In an RGB colour image, each pixel has not just an overall (grayscale) tonal level but an intensity value in each of the three colour channels (so that R225, G225, B0, for example, describes a canary yellow).

The histogram in Adobe Camera Raw displays separate graphs for the three colour channels in addition to the white composite graph, and also combined channels: cyan for green and blue, yellow for red and green, and magenta for red and blue. This enables you to spot clipping in specific channels.

While a loss of tonal detail in one or two channels isn’t as noticeable as blown highlights or solid black shadows, it can indicate areas in which there’s insufficient information to render detail, so it’s worth looking out for.

Darkening vs Lightening

The data in a digital photo isn’t captured uniformly across the tonal range to begin with. Light is recorded in a very ‘literal’ linear fashion by the photo receptors on a camera sensor: where twice as many photons hit the sensor, twice the level of data is recorded.

In practice, darkening tones down is always more successful than the reverse, because there’s more detail in the brighter tones that can be pushed down into darker tones than vice-versa.

This means that after using Exposure for your major adjustments (or Brightness to lighten from the midtones, or both), you can usually turn to the Recovery slider to pull back even more detail from the brightest tones.

Go easy with the Blacks and Fill Light sliders, though, as these affect the darkest tones, where there’s less data to work on.

PAGE 1: Assessing your exposure
PAGE 2: Clipping warnings
PAGE 3: The histogram and colour


How to read a histogram: free photography cheat sheet
Truthful Tone-mapping: a quick guide to realistic HDR in Photomatix Pro