Well, the new Nikon 5D is out, and according to the specs, has a top (boosted) ISO setting of 3,280,000. That's 3.2 MILLION! So..... I has to see just what this meant in real terms. Not having $6500 to try one - and they're not in stores yet - just did some comparisons on a spreadsheet of ISO vs Shutter Speed vs F-stop using the "Sunny 16" rule we all learned when we started back in the film days.
First, I looked up the fastest shutter speeds in a DSLR, and came up with 1/16000 on a few micro 4/3 cameras. The Nikon only has a top speed of 1/8000. Next, I had to set an F-stop range that's actual. I know there are f0.95 lenses out there, so I set f1 as the low point, and f45 as the high. My kit lens only stops down to f32, some of the others to f22, but I do have a manual focus, cheap tele that goes to f45, so it's my high point.
Please understand that I did the Sunny 16 calculations using the original formulas, and let the spreadsheet do the calculations. Therefore, ISO 100 uses a SS of 1/100, not 1/125. Just like my old Argus C3 that I learned 35mm photography on! So you'll see abnormal shutter speeds like 1/12 instead of 1/15. Use your brain to extrapolate to modern values.
You can see from the attached chart that even in open shade - 4 stops down from the sunny values, that you can't even use f45 and 1/16000, the most you could use is ISO 1.6Million.
Of course, this high of an ISO is designed for very low light conditions, but how to approximate for this? That's where the Looney 16 rule comes in! Under a FULL moon on a CLEAR night, there should be 18 stops of difference from the Sunny 16 rule. That still means 4 seconds at at ISO 100 and your f1.4 prime, but now we're getting usable for the high ISOs. According to my chart, you could do an action stopping 1/4000 sec exposure with your Noctilux at ISO 3.2 million.
Attached is a screen grab of my chart.
- (203.36 KiB) Not downloaded yet
An a700 and couple of a580s, plus even more lenses.