Camera makers provide "highlight headroom" to prevent clipped highlights. They could do this by adjusting the metering calibration, but this would complicate the use of hand held light meters, so they provide highlight headroom by adjusting the ISO rating of the sensor. The ISO 12232:1998 saturation standard is the easiest to understand, but is no longer used by most camera makers. However, DXOmark continues to use it for determining the ISO sensitivity when dealing with raw files; for details of the DXO method look here. The Wikipedia article filmspeed has a good section on the current standards and also covers the original 1998 standard.
The 1998 saturation standard specifies that when an 18% reflective target is exposed according to a standard light meter reading, the sensor will achieve 18%/sqrt(2) or 12.7% saturation, which allows 0.5 EV of headroom as compared to 18%. Note that 18% is 2.47 EV below 100%. According to this standard, if you expose an 18% gray card according to the light meter reading, the raw file should have 12.7% saturation, which is 119 in sRGB. One way of placing the highlights of the scene is to take a spot meter reading of the highlights and add about 2.5 EV to the indicated reading.
As an example, I photographed a gray card with my D800e using both JPEG and raw. I rendered the raw file with ACR 9.5.1 using the Adobe Standard profile and PV2012, both with the default exposure and negative exposure of -0.35 EV to account for the BaselineExposure correction that ACR uses for this camera. The results are shown in the table below. The camera JPEG rendered 18% gray at a sRGB value of 153, well above the nominal value of 119. The default ACR rendering was 157 and application of -0.35EV exposure in ACR gave a more reasonable value of . Rawdigger gave a 14 bit reading of 1970 in the green channels, which is 12.52% saturation. This is very close to the sSAT 12.7%. To complete the analysis, one could do an exposure series and note the correlation between the camera histogram and blinking highlights with clipping as shown by Rawdigger.
DXO lists the Manufacturer rated ISO vs Measured ISO of the cameras they test. Values for the Nikon D810 are 64 and 47 respectively and these values for the D800e are 100 and 73; the difference is 0.45 EV (log 100/73, base 2), which is consistent with my data. Values for the Canon 5Ds are 100 and 77 and those for the 1Dx are 100 and 80. Phase One rates the IQ180 at 100 whereas the measured is 29. These differences are to allow for highlight headroom, which seems excessive for the IQ180.
I invite you to do a similar analysis for your camera. Personally, I find the ERADR term confusing and unnecessary. Why not raw headroom?
Alter Ego, DaveEllis, Bart vanderWolf, Bill Janes, Guilliermo Luijk, Erik Kaffehr, Tony W., Tim Lookingbill, and.Lundberg02.,
I have reached the point, that, without taking pause to substantively review all your thoughtful replies that diminished returns may acrue, and I'd rather avoid that.
l appreciate very much the collective serious and professional nature with which my suggestions, considerations, and questions have been met. And I appreciate the patience and..even the palpable forebearance displayed in the face of what must seem to be ... irritating ... persistence.
It will, admittedly, take this field biology researcher cum pathologist some time to digest the wealth of opinion and information with which I've been presented. I am particularly grateful for the nudge to more seriously compare the benefits of ACR and RawDigger/FastRawViewer in the process of ERADR determination. (a couple examples below)
I'm aware that some have an antipathy toward the term ERADR in place of " highlight recovery headroom" but understand that I remain in awe of the ( to me) considerable evidence that significant functional DR persists to the right of the clipping warning of the JPEG histogram frame and, no less significantly, also of the ease of replicability with which the amount of ERADR can be documented from camera-to-camera in amounts ranging from 1/3 EV to two and 2/3 EV ( at least). At least for that purpose I'm not sure that ACR and FRV are not of reasonably equivalent utility, but, thanks to the cautions expressed by we've real of you will be further pursuing that possibility as I become more facile with this return visit to Raw Digger.
I do, sincerely, thank you all for your patience in expressing obviously strongly held and well-considered opinions. And Bill's topic summary is also appreciated.
That brief return visit - guided by Guillermo -to the Cave Allegory reminded me of its need to be considered within the context of assumptions associated with two or three(?) other allegories before the Dialectic can ultimately be adequately appreciated and internalized.
I am confident we'll rejoin this topic at some future date, but until then, be assured I value the perspective you all have provided. This has been, for me at least, a most stimulating thread.
Dave Graham / uuglypher
East River, South Dakota