Hi Guillermo,
Thanks for commenting. I understand, but disagree with with your conclusion that users
of EBTR operate under the mistaken "...assumption that this is an accurate calculation that will provide perfectly ETTR'd RAW files under any situation." Whether you are referring to the calculation of one's camera's allotment of extra raw-accessible DR (ERADR) or are referring to determination of the exposure at which jpeg highlight clipping will occur (when the determined ERADR must be added to the exposure to accomplish EBTR) is unclear. It is essential that both be accomplished accurately to assure utilization of all available raw-accessible dynamic range.
However, the explicit statement is that the EBTR process as described will fail to result in "...perfectly ETTR'd RAW files under any condition." "Perfect" accomplishment of the entire available raw-accessible DR? Well...O.K....I'll admit that we work within the slop factor of something less than 1/3 stop ...which we find to be adequate on a practical basis but which you may consider to be an error level of insufficiently "fine grain". In fact, some practicing EBTR
users actually ignore the last intact 1/3 stop of determined ERADR and use it as "cushion" and a ( likely unnecessary) guaran-damn-tee against any clipping of highlight detail in the raw image data. There is no question that ACR does an amazing job ( at least to my mind...) of recovering image data in the event of clipping in one of the color channels and I am not unduly concerned when FRV informs me of that happenstance. Too uncritical? Perhaps guilty as charged, but it is so rarely (ever?) of practical significance that I relegate it to inconsequentiality.
Now, I have long suggested that EBTR is really a simple procedure, but your statement:
"It simply consists of correlating the camera JPEG blinking lights with what you find later at the RAW development stage, to try to figure out an estimation on how much you can allow your JPEGs to clip before the RAW file actually begins to clip as well." is a touch too misleadingly simplistic.
The key to successful use of EBTR is to realize that cameras vary not only in their maximum raw-accessible DR, but also in the accuracy of their histogram, correlation of the right end of the histogram frame with clipping, and/or correlation of the onset of the "clipping blinkies" with the actual onset of highlight clipping of a JPEG file. However, your salvation is that when you
identify a given camera's most accurate indicator of actual jpeg clipping, THAT remains consistently accurate, and it is at that point that one adds the determined ERADR of that camera to accomplish EBTR. .
I have two cameras that both provide one and 2/3 stops of ERADR.
In camera A the accurate indicator that highlight clipping will occur with the next additional 1/3 stop of exposure is the first definitive climbing of the right end of the histogram up the right side of the histogram frame. (at which point the onset of "blinkies" has not yet occurred).
In camera B the first indication that highlight clipping will occur with the next additional 1/3 stop of exposure is the onset of the blinkies ( which occurs immediately after the histogram' right extreme barely "kisses" the right side of the histogram frame but before it begins its climb
up the right side.
With another camera that provides one full stop of ERADR I have found that spot-metering
the brightest finely detailed highlights (subject Value VIII) and adding three full stops arrives at the point of jpeg clipping (which is past the point of the onset of blinkies).
A good friend spot-meters detailed highlights (value VII) , adds two stops, and then adds his camera's two stops of ERADR.
These indices of the onset of jpeg highlight clipping, although at great variance with each other, are consistently reliable in each camera.
The unavoidable axiom of success with EBTR? "Know your camera !"
There are a number of benefits that accrue to use of EBTR as I've decribed the practice, and
one that is often forgotten is the decreased frequency with which it's practitioners find the
need for HDR, particularly those whose cameras have one and 2/3 stops or more of ERADR. Happily, my two work-a-day cameras come in just at that line! Although some decry its presumed lack of precision, experienced users of EBTR routinely rely on its demonstrated accuracy and repeatable reliability...and were that accustomed precision found to be
unreliable, EBTR would quickly fall from use.
Is EBTR useful under all conditions? It offers complete reliability only when the scene DR
is exceeded by the camera's full, raw-accessible DR at the intended ISO.
Again, Guillermo, I offer explanation and attestation to the utility of EBTR not as a proselytizer, but simply to present a practical alternative to the presumed necessity of a more technical approach to assuring proper exposure of raw image data. Seriously interested photographers will try both approaches and make their choice. and I suggest that use of one path rather than the other is a choice born of personal experience and is not, by any means (as I have heard) an act of faith nor a character flaw.
Regards,
Dave Graham