Originally posted by bart_hickman I'm pretty sure the 0-255 range doesn't map linearly to number of photons--it's a higher order mapping. So 128 is a lot more than 1 stop below 255--probably more like 2 or 3 stops.
The RAW image (12 bits) might be linear.
It's a little more complicated than that. SNR due to photon quantization isn't varying linearly with number of photons. It improves as the square root of photon count. There's also readout noise portion of SNR--that does go down directly with number of photons. The total improvement of noise will be somewhere between the two. As you get closer to the shadow end, readout noise becomes more significant and so you might observe SNR in the shadows almost increasing linearly with number of photons.
Doesn't change the correctness of your conclusion--ie., for lowest SNR, the farther to right you expose (without blowing highlights), the better.
Are you sure the camera even uses analog amplification to achieve higher ISO? At least on the K10d, I'd think with a 22-bit ADC and 16-bit datapath, there'd be no need for analog amplification.
Anyway, if it does use analog amplification, then you're right, you should theoretically get better SNR at higher ISO for equal exposure settings (f-number and shutter speed)--although the difference may be negligible if the noise from the sensor is already much larger than the ADC noise or other noise sources after the sensor. If it doesn't use analog amplification, then there will be no difference.
In my experience with the K10d, ISO1600 EV0 is less noisy than ISO800 EV-1, but it's also pretty obvious that in-camera NR has been applied--even to the RAW data. If you run both images through auto-profiling NR software to put everything back on a level playing field (eg., Noiseware), they come out looking virtually identical--both in terms of noise and in terms of detail retention.
Bart
Bart, no argument and thanks for the additional detail.
Yes, I was over-simplifying for the purposes of clarification of the concept. I thought I was clear about that, but maybe not.
In practice, if you keep the concept in mind and apply it as often as possible, you will get images with the best possible use of the DR of the camera regardless of the technical reasoning behind it. I use this rule of thumb along with a modified zone system quite often where I spot on white and add 1.5 stops to the exposure and then chimp to see if I am as far right as I can get without going over. I shoot only RAW and this works pretty well for me, assuming that there is any white in my scene
Black works just as well, but in an inverted way (subtract 1.5 or so stops).
You are correct about the noise behavior as well, but I think that the person I was responding to had already stated that the technical reasoning of some of these rules of thumb was not something he understood or perhaps not something he wanted to understand that much. Frankly many folks cannot, or do not want to, apply such specific technical information to their regular use of the camera and therefore I do not think such details are all that helpful. This is not a slam against any of those folks, as it is not necessary to understand any of the specific details to get good images. After all, how much did we know (or care) about how film emulsion was made and how every layer worked and responded to light? Most of us just came to understand how the film responded to certain situations and then adpated our techniques to get the final image we wanted.
Are the details correct? Perhaps, assuming that they are not based upon guesses about things we do not know for sure, but they are sometimes hard to translate into taking pictures and specific results. Expose to the right is much easier to understand and you can see the results directly when PP'ing the images.
Many parts of the signal path in the K10D (perhaps almost all) run directly through the NuCore chip, which has many features that Pentax may or may not have chosen to use/implement, so it is not easy to decide exactly what is going on, and honestly, it really does not matter as it is not like we can change it. It is interesting to talk about, however.
I do not think anyone has figured out for sure if there are analog amplification steps in the K10D system design. However, this propaganda document form NuCore would lead me to believe that the gain is adjusted in an analog section which is handled by the chipset:
http://www.nucoretech.com/nu3/images/80_downloads/pres_technology.ppt.us.pdf
Lastly, I purposely did not state that I agreed with the "shoot higher ISO to the right" versus shooting a stop lower as I have not really made up my mind whether there would really be any noticeable difference in doing this or not. I have also not taken the time to test the idea on the K10D as I do not recall ever shooting that camera above 400 ISO. Things just get too ugly for my taste above that, so I reach for the K100D in those situations.
Thanks again for the added technical bits.
Ray