@Garry23
I experimented a lot with your HHB method trying to figure out if it provides any advantages compared to the Dual ISO method. My conclusion is that if there are any quality differences between both methods they really are very subtle. The HHB method really provides very clean, almost noiseless results but I struggled a lot trying to handhold all 3 (even 6) shots in such manner that non of them suffers from camera shake blur. If even one of the brackets is blurry, you notice blurry areas in the final picture also. The Dual ISO method seems to be more forgiving in that respect since you take only one shot and the final result depends entirely on that one shot. If exposure time starts becoming critical for hand holding, I usually take 3-4 identical shots and chose the sharpest one, discarding the others. With stabilized lenses I can go down to 1/4-th of a sec. and still have perfectly usable results.
The Dual ISO method also has its caveats, compared to the HHB method:
1) Theoretically, it provides reduced resolutions in the darkest and brightest areas of the shot. In practice, it is extremely hard to see its effect visually even if pixel peeping at 100%. I saw the difference only once, with a high-contrast interior shot, where the transparent curtains, partly covering the windows, were made of a very fine meshed material. At some locations, the horizontal threads of the mesh looked broken, obviously due to the Dual ISO effect at the resolution limit. To notice this phenomenon however, one really needs to know where to look and what to look for.
2) It requires precise ETTR metering for the highlights, especially in high-contrast scenes. If you overexpose them, the shot is gone. If you underexpose the shot, trying to preserve the highlights, you may end up with some noise in the darkest areas due to reduced single-to-noise ratio there. This again happened to me with a contrasty shot taken at Dual ISO 100/800. When I changed the setting to 200/1600, the noise in the darkest areas disappeared almost completely and I had perfect detail and sharpness over the entire area of the picture.
Bottom line:
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My choice for handheld low-light photography is the Dual ISO method. The risk for ending up with blurred shots as a result of camera shake is much lower, so is the post processing time. In terms of image quality, dynamic range, colors and detail, the Dual ISO method provides excellent results but one needs to be precise with exposure to the right. With tools like RAW-histogram and zebras, Magic Lantern makes this task easy.