Audionut is challenging it, so we don't have a final word on this.
I am? I corrected myself in the ADTG thread. On the 5D3, ISO 3200 is probably the last
useful ISO. I've
posted a few
examples now that show this.
If you mean that last data sample of mine on saturate offset, I'm not confident either way, just that the data was showing strange results.
---------------------------------------------------------------------
If that last 0.1-0.2 EV of shadow detail is more important then 1EV of highlight detail, you
may benefit from ISO 6400 vs ISO 3200. And while the data suggests that ML ISO 6400 can give 9.1-9.2 EV of dynamic range, it does not consider the other sources of noise inherent in digital photography. Bear in mind that the dynamic range data provided by most sources (including ML displays), is basing this figure on the difference between the maximum saturation level (white level) of the signal being captured and the
noise floor. It doesn't care how noisy the signal is between these 2 points. This is where SNR curves are useful, as they display the SNR (image quality) through the entire exposure. A general assumption for
usable dynamic range, where the shadows aren't concealed by an excessive amount of noise, is to subtract 2EV from rated values (and this is subject to processing techniques and viewer tolerance on image noise). If we apply this to the data we have for ISO 6400 on the 5D3, we are now left with around 7EV of useful data. Suddenly that 1EV of highlight data that is getting thrown out when boosting ISO another stop (ISO 3200 vs ISO 6400 in this example), might actually be useful. We've got 8EV of usable dynamic range with ISO 3200 vs 7.1EV of useful dynamic range with ISO 6400, and we haven't even considered the noisiest source in the signal yet, the shot noise.
At these low light levels (where ISO 3200/6400 is exposed correctly), the dominant noise source is the light itself (shot noise). This noise in our images is controlled entirely on the number of photons (the amount of light) hitting the sensor. As I like to point out at every opportunity,
exposure (light hitting the sensor) is only controlled by lens diameter, shutter and aperture. ISO
does not control the light hitting the sensor.
Bumping ISO does not effect the shot noise of the image, period.
Analog ISO (voltage gain) is useful for boosting the the signal from the sensor. The voltage gain is applied
at the sensor. So all those little bits of electronics further down the chain (ADC, DAC, etc), that have
their own inherent Signal to Noise ratio, receive a full signal (from the sensor), and retain their full SNR capability.
Analog ISO actually reduces the noise from the camera electronics by boosting the signal
through all of the remaining electronics and increasing the
sensitivity of the amplifier*. Random noise and Fixed Pattern Noise (banding noise), are reduced with
increasing ISO. The apparent noise increase with increasing ISO is that pesky shot noise that ISO
does not control.
edit: Readout noise is also increased with gain, but is also dominated by shot noise at low exposures.
The reason why we gain any improvement at all right near the noise floor for increasing ISO,
where our images are shot noise limited, is because the camera electronics are still dominating the total noise near the noise floor.
*Note: Rodger Clark specifically states that ISO does not increase the sensitivity. He is referring to the sensitivity of the sensor.
-------------------------------------------------------
If you want to conduct your own testing, I would suggest that you expose your minimum test ISO at maximum saturation. This ensures that the base line exposure contains the lowest amount of shot noise. In this way, your testing procedure contains controlled shot noise for the tests. *see notes below
Remember, if you have to reduce exposure when increasing ISO (because you're tossing away 1EV of highlight data with every stop), you increase the shot noise with the exposure adjustment, and ISO does not reduce it.
Throw in tonal range with
exposed bit depth, and you're on your own!
*Note: You should not change exposure settings with increasing ISO. We know the highlights will get clipped by 1EV, it's the shadow detail that important in this test.
