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Subject: Re: Exposure to the right and tone placement



Rags Gardner said:

"A digital sensor is simply a photon counter as explained. And raw values represent linear voltages as explained. And perception is not linear as explained. However image tones and binary encoding have nothing at all to do with each other. This is being denied by many “respected” photographers and unfortunately blindly accepted by too many others."

Rags, I certainly appreciate your input in this forum and recognize that you know something about computers, having recently retired from a career at IBM.

Let's concentrate on a topic we can all agree on: digital sensors are photon counters. The consequences of this property are discussed in a very informative post by Roger Clark, MIT PhD in astrophysics and an accomplished photographer. In summary, when photons shrike a photo sensor, electrons are released and accumulate in what can be likened to a well, and result is a voltage, which is read by the ADC and digitized with a bit depth of 12 in most cameras. When the well is full, the sensor is saturated and the full well capacity of a Canon 1D Mark II is about 52,300 electrons. Full well is attained when the camera is used at ISO 100. If you use ISO 400, the full well is not utilized, but rather the gain is increased to attain the same voltage and noise is increased.

The standard deviation of counting is determined by the Poisson distribution and is simply the square root of the count. If you double the count, the error is reduced by a factor of 1.4, not halved and you reach a point of diminishing returns.



The table below is adapted from Roger's calculations for the Canon 1D Mark II with electron counts and noise levels shown according to Roger's methods for various Zones. The shadow noise and the number of usable levels are more related to the laws of physics and statistics than the digital encoding (presiming you use enough bits).



I am a proponent of exposing to the right, but one must be aware of the dangers. If you double the number of photons by giving another EV of exposure, you decrease the noise by a factor of 1.4 and gain something in the shadows, where noise is critical. I will leave it to the experts to determine the number of levels gained in the shadows.

However, if you inadvertently blow one stop of highlights you lose 2048 levels according to the prevailing wisdom for a 1.4x gain in shadow noise. Is this wise? In my experiment, I gave an additional 0.5 EV of exposure over nominal and lost the highest 0.1 density (1/3 stop) of highlight detail for an insignificant gain in shadow noise. Which is better? Highlight recovery in ACR is great, but is of no use if all 3 channels are blown.

These results are very similar to what Rags noted in his tests and confirm his conclusion that there is not much headroom above Zone V with digital. For those who insist on Zone IX exposure, just use proper placement.

Here are plots of the linear 16 bit images analyzed by Norman Koren's Imitest. Linear is used to avoid changes produced by gamma and rendering. Yes Jeff, I am aware of linear encoding, and no Bruce, your black hole in not demonstrated. :)




I have no financial interest in Imitest, but am merely a user of the program and a fan of Norman Koren (who also has weighed in on exposing to the right).

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