Originally posted by Mark Ransom I would agree with your first sentence - the base ISO is the point where you've achieved the maximum dynamic range, which is determined by the signal/noise ratio. Assuming the noise floor is constant, it would be the point where you have the maximum amount of signal before overloading the pixel cells. That would certainly be a technical limitation of the sensor.
My understanding of ISO invariance is different, it's all about increasing ISO. It means that increasing the ISO is no different than simply turning up the brightness during the conversion of the RAW image.
I haven't seen any evidence to suggest that's even possible.
It would only be relevant to an image that would use it's full dynamic range. If an image requires 9 EV DR, and your seniors produces 15 EV, then you can expect a lot of loss of EV before the loss of DR becomes visible. All the images I've seen in support of ISO invariance didn't stress the DR, as almost every sunset image or image with bright sun and depp shadows does.
So in that sense it appears to me to be slight of hand. Use narrow DR image and you can convincingly argue there's such thing as ISO invariance. Using images that are improved by more DR or degraded by the lack of it, and I'm guessing ISO invariance disappears. But as I said, it's always appeared to be nonsense to me so I pretty much ignore it.
I just always shoot at the lowest ISO possible, if that's no longer necessary, someone should let us know. I find it hard to believe a 400 ISO image looks the same as an under exposed 100 ISO image bumped up 2 stops in post.
Has anyone actually done this?
The only examples I've seen showed distinct change in contrast values, which the author conveniently ignored in arguing for ISO invariance. IF it's not exactly the same image, there is variance. Not only that the author didn't seem to understand the effect on contrast of sliding the curves towards the left, making the whole concept meaningless.