This morning I was reading a couple postings where I encountered some of the usual Internet confusion about what should be an easy to understand concept-- Dynamic Range. I was about to toss in a few words about my take on the subject when I decided to repeat an experiment I did 10 years ago.
Then I had just bought my first digital, a 3mp Oly 3020. P&S. A high end camera in those days. And being an optical mechanical engineer and early pixel peeper I wanted to learn how the camera's innards worked. Like how many levels there were in the camera's A/D. I asked in the only real photography forum around, rec.photography.digital on Usenet, Since nobody seemed to know, I worked out a simple experiment so I could enlighten the Usenet world and start down the Path to Usenet Guru. Ah, a simpler world then.
The experiment: Sent up a target and a find an exposure where the brightest area was starting to clip. Then keep taking pictures as you reduced the exposure until you end up with just noise. That should be at or near the lowest levels of the A/D. And though I wasn't thinking about dynamic range at the time it gives you that number.
I got fancy back then and used a table loaded with stuff. Today. I attached a print of an excellent test target to a bed desk (It can be found on the Internet but I don't remember its name.) To that I added Charlotte's collection of washable markers and a paint swatch. (Home Depot paint department, White on White to Ink Drop)
The main image in the screen shot was taken at 1/13sec, f5 and ISO800--3 stops away from ISO 100 which uses almost all of the levels of the D7000's 14 bit A/D. The smaller image was shot at 1/5000sec, f10, ISO setting of 6400, for a 7 1/2 stop under exposure to give an ISO equivalent of 1,000,000.
So, believe it or not, you are looking at a recognizable image taken 13.5 stops off the "correct' exposure.
And believe it not, I did. That image was too good with too wide a range of tones. So I did everything all over again with the same results.
After some research I believe several things are going on. The R G and B channels all have different gains, something I could see using the channel mixer. Noise dithering is smoothing out the tone steps I expected. And both Photoshop and Irfran have some sophisticated algorithms that allowed me pull out the detail in post processing.
As for the post processing, With the really low light images you start with a black jpg and end up with a black conversion even with the exposure slider in ACR set at 4X. Bring what you have into Photoshop and add a levels layer. Set the boxes on the layer lever to 0 for black, 4 for middle grey and about 50 for the end of the mostly noise histogram. The posted image still came out dark so I adjusted the gamma slightly in Irfan when I resized it for posting. This also improved the S/N since Irfan bins pixels when it resizes an image
Finally the image at the bottom was taken at minus 14.5 stops into a 14 bit A/D. As expected it shows all noise. Or almost, since there was some traces of image detail in the red channel due to the different gains.