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Subject: Re: Exposure to the right and tone placement



Jeff,

How would you expose for a picture of a black cat on a dark background? If you place the exposure too far to the right so that the lightest tones are just short of clipping, the tones will be distorted by the expansion that takes place when the image is rendered from the raw as the gamma curve is applied (as shown by Rags' histograms) and any s-curve applied by the rendering will be applied to the wrong tones in the image. Anyone who exposes in this fashion does not understand linear capture.

In the above case, a gray card or incident light reading would get you closer to proper exposure. Since the subject is unusually dark, you would probably open up 0.5 to 1 stop as suggested by Kodak. Alternatively, you could use the zone system approach and place the cat at Zone III or so. As explained by Adams, once you PLACE one zone, the others FALL into position. Actually, you can use any zone for placement and the others will then fall into place.

In what Adams refers to as a short scale subject (where the dynamic range of the subject occupies only a portion of the linear portion of the characteristic curve), there is some lattitude in placement. For negative film Adams placed Zone III at Zone III (rather than Zone IV) so as to have less grain and higher acutance in the negative.

With positive film or digital most of us try to place Zone IX at Zone IX so as to make best use of these media. Of course, with film some photographers slightly underexposed Kodachrome and others exposed their Velvia at ISO 40 rather than the nominal value of 50 (O.3 EV overexposure) in order to make best use of the characteristics of the medium. Once the characteristics of the mediam are taken into account, I don't really see that much difference in exposing for film or digital. We digital we could place Zone V at Zone V, but any misjudgment would risk clipping of the highlights. As Bruce Fraser has pointed out, current tools are inadequate for really accurate placement and we are left in somewhat of a quandry.

Rags,

Your comment about the camera histogram being based on a rendered image is key, since the rendering does more than simply apply a gamma curve--it reduces the dynamic range of the scene to a lower value that can be realized in a print or on a computer screen and maps the tones so as to achieve a pleasing rather than literal interpretation of the linear tones. For interested readers, Kodak discusses this matter briefly in their white paper for ROMM_RGB (ProPhoto RGB).



If we really want to know what is going on with the raw image, it is best to examine the unrendered image. That is why I used DCRaw and a 16 bit linear space for my tests.

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