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giovedì 13 dicembre 2012

Exposing to the Right

Exposing to the right (often refferred to as ETTR) is a technique that seems to polarise opinions across the internet so you can find plenty of examples of people supporting its use and an equal number of people claiming it adds no value. The principles of the technique however do hold value and are valid to consider when out taking images. As it is a technique that I regularly employ when photographing landscapes I wanted to share the reasoning behind it, and show an example of the benefit it can bring. 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, maximising your image quality.
Let’s consider the CCD or CMOS sensors found in most digital cameras. Typical DSLR sensors can capture seven stops of dynamic range and produce 12-bit raw image files, capable of recording 4096 tonal levels in each red/green/blue channel. The ability to record such a large number of tones should guarantee smooth transitions between the tones within the resulting image, however it is not quite that simple. Whilst you might think that each of the seven stops in the range of the sensor record an even number of tones throughout the dynamic range, you would be mistaken. F-stops are logarithmic in nature meaning that each stop records half of the light of the previous one. Practically, this means that the brightest stop records half of the possible number of tones, i.e. 2048, the second stop records half again, i.e. 1024, and so on until the seventh stop that records only 32 tonal levels. Therefore, if you underexpose an image and correct the exposure during in post processing, the tonal transitions in the darker areas will not be as smooth, and the risk of degrading your image quality is much higher. If you overexpose your image, by pushing the histogram to the right, you will capture much more tonal information that results in much better image quality when correcting the exposure in post processing. Read more...............