For a number of years specialist websites have been promoting a technique that involves adjusting the exposure to place the histogram peak as far as possible to the right of the graph without clipping highlights.
Theoretically, Expose to the Right Technique (ETTR) should get the optimum performance out of the image sensor and deliver a wider tonal range in dark areas, greater signal-to-noise ratio, fuller use of the colour gamut and greater latitude for editing.
In practice, it’s not quite that simple; some cameras aren’t optimised for ETTR, particularly those that use lossy raw compression. There are also some preconditions that must be met:
1. ETTR works best with raw files so the histogram should be used as a guide only since it is based on JPEG data. (Raw files should have a slightly wider dynamic range than JPEGs.)
2. The technique only works at base ISO (the lowest value in the native ISO range, which is normally also the default ISO setting in Auto ISO mode).
3. Set the exposure level using the camera’s histogram and/or the highlight clipping displays. Some cameras include a highlight metering option, which can be useful.
4. When the dynamic range in the scene is wider than the sensor can record, expose for the most important tones and sacrifice the least important ones.
5. Bracket exposures across a range from -1/3 EV to +4 EV, based on the metered value for the brightest point in the scene, using spot metering. This will provide some protection in case you accidentally expose too far to the right.
6. Avoid ETTR with low contrast scenes. It’s not necessary and you’re likely to end up with an exposure that is too dark. (With contrasty scenes, the camera’s meter could do the opposite, recommending an exposure that is brighter than ideal.)
The upper image was taken with the camera’s metered exposure. Chimping showed the histogram was skewed to the left, clipping shadow areas. Increasing exposure by 2/3 of a stop pushed the histogram to the right, opening out the shadows and brightening the lighter areas in the image to deliver a wider dynamic range. The small amount of highlight clipping had no significant effect on the overall image. Both shots were captured as raw files.
With the wide dynamic range capabilities of modern sensors, as well as a much lower amount of noise at base ISO, ETTR is becoming less necessary. Today’s sensors make it easier to recover shadow details, giving us more legroom to recover data in post-processing. But if you’re using an older camera, ETTR can be a valuable aid, provided you’ve captured uncompressed raw files and the picture is successful in every other way.
Article by Margaret Brown – see Margaret’s photography pocket guides