Online video tutorials can give the impression that shooting at anything but base ISO is somehow detrimental to image quality; and tests evaluating underexposure latitude in post are almost without exception done in SDR. However, because so much of HDR’s dynamic range is located in the shadows where noise tends to be most prevalent, and as noise is far more distracting in HDR than in SDR, latitude tests as currently done don’t tell the whole story and reducing ISO can substantially improve picture quality.
Art Adams explains:
CUT THE CAMERA’S ISO IN HALF
This is something I do habitually, as the native ISO of many cameras tends to be a bit too optimistic for my taste, and I’ve learned that Bill Bennett, ASC, considers this a must when shooting for HDR. Shadow contrast is so high that normal amounts of noise become both distinctly visible and enormously distracting.
Excess noise can significantly degrade the HDR experience. The best looking HDR retains some detail near black, and noise “movement” can cause loss of shadow detail in the darkest tones. Rating the camera slower, or using a viewing LUT that does the same, allows the colorist to push the noise floor down until it becomes black, while retaining detail and texture in tones just above black.
And here are some examples of how he rated his cameras’ ISOs (2015):
Alexa: 400 instead of 800
Sony F55: 640 instead of 1250
Sony F5 and FS7: 1000 instead of 2000
RED Dragon: 400 instead of 800