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Subject: Re: Exposure to the right and tone placement


Of course I agree entirely with your approach, having learned most of what I know from your writings--several editions of Real World Photoshop, the two ACR books, and Real World Color Management (2nd ed) as well as the the CreativePro series and other posts.

I agree (if anyone cares) that one should expose for the highlights and this requires measuring them. On the step wedge A (highlights) and M (mid tones) are seven steps apart (density difference of 0.7 or 2.325 stops). Meters are calibrated for M (more or less). In exposing for the step wedge we could take a reading from either. However, with a real scene, M is not marked or easy to judge and it is best to take a highlight reading to place the highlights.

This post discusses the lossy compression of the D70:

"The quantization discards information by converting 12 bits' worth of data into into log2(683) = 9.4 bits' worth of resolution. The dynamic range is unchanged. This is a fairly common technique - digital telephony encodes 12 bits' worth of dynamic range in 8 bits using the so-called A-law and mu-law codecs. I modified the program to output the data for the decoding curve (Excel-compatible CSV format), and plotted the curve (PDF) using linear and log-log scales, along with a quadratic regression fit (courtesy of R). The curve resembles a gamma correction curve, linear for values up to 215, then quadratic.

In conclusion, Thom is right - there is some loss of data, mostly in the form of lowered resolution in the highlights."

In my experiment, I was measuring dynamic range. Highlight details may have been lost but I was not measuring gradations within the levels. Also, the principle applies to lossy or non lossy storage. Those fortunate enough to have a professional level camera can do their own tests, but for now I have to make best use of what I have.

Nonetheless, the point of my post discussing digital counting statistics (and on which you made no comment) was that doubling of exposure decreases noise by only a factor of 1.4, whereas blowing the highlights can cause a loss of thousands of highlight tones (if your camera does not have lossy compression). Therefore, exposing to the right should not be carried to far. As most have agreed, putting theory into practice is hampered by the lack of proper metering devices, but nonetheless we get pretty good results with the current technology judiciously applied. Not everyone will use the same approach.



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