Since the longest shutter speed is 2.0 sec, what will the final image look like? A true 10 minute exposure, running water will in many situations totally disappear and almost turn to smoke, but since the longest single shutter speed is 2.0, what does 1200 frames averaged together appear as?
The
raw you speak of had a fast per-frame shutter speed of 1/125 which means a sharp capture of the water followed by large gap between each capture, repeated many many times. That kinda-sorta equals out to smoke, but if you look carefully you can see where the water is many many many many many sharp waters blended together. That is subtly different than a single long exposure.
So again... if the shutter speed is relatively slow and renders motion in a blurred-per-frame-way with no gaps between frames the effect will be
identical to one continuous uninterrupted exposure in terms of motion/subject, but with far less noise in the shadows (even lower than the already very low noise of a normal IQ4 capture).
If the shutter speed selected is fast and sharply renders moving objects with large gaps between frames (e.g. 1/125) then you you'll get something similar, but not identical.
Each individual frame of the averaging is exposed the same as a normal frame. Though, since the light may change while these frames are being captured, and because you don't want any pixel to blow out if you can avoid it, it will likely be deemed best practice to underexpose by a stop compared to if you were doing a normal capture. Considering this technique makes the shadows several stops cleaner, the net result should still be a significant increase in usable dynamic range.
Yep. It's a long exposure technique. So if your scene and desired aesthetic won't tolerate a long exposure technique (or a long exposure + single frame + compositing; which as you note sometimes is workable and sometimes is not) then Frame Averaging is not for you.