Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
Jack Hogan Veteran Member • Posts: 6,709
Re: Questions

ArvoJ wrote:

completelyrandomstuff wrote:

This is why DXO reports the S/N ratio at 18% btw. This 18% corresponds to 50% gray.

Jack Hogan wrote:

Does 18% of saturation in the raw data correspond to middle gray in most DSCs?

Offtopic, but raised interest I found some blog entry, which IMO explains some origins on 18% and 50% equivalence:http://stonerosephotos.com/blog/2010/02/more-on-18-gray/ No connections to specific equipment of course.

Hi ArvoJ,

Of course. Middle Gray, or L*50, is indeed 18% of 100% diffuse white (or 2.5 stops below it). On the other hand the vast majority of current DSCs tend to spot meter a middle gray card so that it will result in raw values of around +/-10% of saturation (or around 3.5 stops below it). Newer ones even less. Have you checked what your camera does? Mine is around 8%.

Many people here can speak to this issue better than I can. But the jist of the reason for the above is that, with very few exceptions, ADCs respond to incoming light linearly for all intents and purposes. And if middle gray were indeed placed only 2.5 stops below saturation a lot of desirable highlights could be inadvertently blown, making DSC owners unhappy. Film with its gentle rolloff on the other hand ... but I digress.

Hansel Adams placed middle gray in zone V and 'pure white' at zone X. So he thought that there were at least 4+ stops of potentially useful photographic information above it, as any landscape photographer will confirm depending on the scene. That would place middle gray at 5% of saturation or less to be able to capture the 4+ stops of detail above middle gray into the raw data (linearly). But most commercial cameras until about 2008, with useful DRs of only 8 stops or so, simply did not have the 'room' to be able to afford dedicating so much of it to tones above middle gray: the shadows would have been woefully noisy. So they compromised: they typically stuck middle gray around 3+ stops below saturation and clipped some of the potentially desirable highlights - but at least they had decent looking shadows.

As sensors improved, middle gray creeped down: to typically 3.5 stops and beyond. Camera makers came up with systems like Nikon's ADL that, depending on an evaluation of the dynamic range of a scene, would 'underexpose' by up to one stop to fit a more pleasing DR blend into the linear Raw data. Underexposed compared to what? No one knows. They simply make 'better' use of the linear dynamic range available to the camera, making their owners happier.


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