RAW headroom: why it matters
In the Dynamic Range section of our DSLR reviews we usually look at something that we call ‘RAW headroom’. The RAW headroom could probably be described as the highlight and shadow detail that has been captured in the RAW data but gets lost when a camera’s internal software applies a tone-curve to the RAW image data.
As a rule cameras apply a typical 'S' shaped tone curve to JPEGs to give a visually appealing contrast without harsh clipping of the brightest or darkest tones (the 'shoulders' of the S curve provide a more gentle roll-off than a straight line would). The tone curve applied is often quite steep, sacrificing the tones at the the extremes of highlight and shadow in the pursuit of 'punchy', consumer-friendly out-of-camera results. At the shadow end this is rarely a problem (the very darkest tones are mapped to black, which isn't visually unpleasant and helps reduce noise), but at the highlight end it can mean that what should be a very pale blue sky turns white, or that highlights have harsh edges instead of a gentle gradient. There's no way to get back detail that the camera's tone curve left out of a JPEG, but it is possible to tweak more out of a raw file. Having access to the sensor's full dynamic range also means that you can fix mild under or over exposure by reaching into the headroom.
And so it's very useful to understand what raw headroom is and how you can make use of it to get the maximum out of your images. We regularly receive questions regarding this particular section of our reviews via our feedback system and in the forums, so I thought it would be a good idea to provide some detail about the dpreview approach to raw headroom in a blog article. This isn't a deeply technical post; it's more of a primer on what we do, and why you need to know about the dynamic range hidden away in your raw files.
