Is there anyone that can answer this simple yet complex question
what is the best ISO in regards to sensor output/dynamic range
I shoot ASA 200(old school) is 100 better...?
Typically the native ISO of the sensor is best because the S/N is higher. That is usually the lowest normally accessible ISO setting, i.e., not an expanded low or high setting. However, some cameras are known to "pull" the lowest ISOs, in which case a slightly higher ISO would be better. Also, some sensors do not employ 1/3-stop variable amplification across the ISO range, so the full-stop ISO settings (100, 200, 400, etc.) are desirable.
Sensors don't have a single native ISO. The gain on the sensor is turned up and down for most ISO settings. The assumption is that turning the gain all the way down (ISO 100 in most cases) would give you the best result, but there was a pretty good test done some time last year on the 1DMIII that showed ISO 200 might give a better result.
ISO 50 (or L) is the gain turned all the way down plus some firmware magic. It is that magic that causes you to lose some dynamic range at ISO 50 on most Canon cameras. I have no idea about Nikon.
My experience with the 30D and the XT suggests that ISO 100 is
best for low noise and high dynamic range. I could believe that ISO 200
would be better for some sensors, though. You'd have to check on a
model by model basis.
I've heard ISO 200 is best on a 40D, someone did tests apparently? Of course I may be wrong, or that person's test may have been flawed. Haven't tested it myself. How would you test it? I wouldn't know where to begin.
I expect the difference between ISO 100 and 200 will be minimal. However, it might have been Emil (ejmartin) who did some comprehensive analyses and published them a while back. I'll have to look for the link.
Here you go. Look at this test for the 30D - but if you cannot stand the technical stuff then the answer you're after is ISO 100. Or 200 The dynamic range is slightly greater at ISO 200 but so is the "dark noise", which means that despite the additional DR there is still more noise in the shadows.
Of course, if you shoot jpeg instead of raw then you've discarded much of the useful DR anyway and it won't matter so much which ISO you use.
I haven't found the link to Emil's stuff yet. It's probably staring at me from the screen but I've lost it. Still, it had some very complex and detailed work along with some interesting and understandable conclusions. I'll look again.
The dynamic range is slightly greater at ISO 200 but so is the "dark noise", which means that despite the additional DR there is still more noise in the shadows.