Ray, can you expain what lead to this "histogram three stops too far to the left" problem?
I agree that one cannot alway use manual setting of shutter speed and aperture, so I suppose you were in a mode like aperture or shutter priority, but then how did you end up with those very low raw levels? Was it shutter priority plus ISO 200 leading to hitting the maximum aperture available?
I was in full manual mode regarding aperture and shutter speed. I had adopted the principle of 'F8 and be there', because I was
there. The shutter speed happened to be set at 1/200th because my earlier recent shots, with 24-120/F4 zoom attached, had been taken with the lens fully extended at 120 mm, which is 180 mm in 35 mm terms. From experience I've found that a shutter speed of at least 1/FL(35mm), in combination with Image Stabilization, is necessary to get good sharpness from very high-resolution sensors such as the D800 and D7100, when the cameras are hand-held. Some folks might disagree with this. I suppose it depends on how steadily one can hold the camera. I have no signs of Parkinson's yet.
Whilst I can't remember my precise thoughts when I took this particular shot (one of thousands taken during a period of a few weeks), I always pay particular attention to the exposure indicator stretched along the bottom of the viewfinder, whenever I shoot in manual mode. I would have noticed that a 200th at F8 was resulting in significant underexposure. I would have been reluctant to reduce exposure even further by using a shutter speed of 1/400th, and reluctant to reduce DoF by adjusting the aperture to F5.6.
Anyway, you surely realize that we are not yet dealing with completely "ISO-less" cameras, and with recent Nikons, there is something to be gained by extra amplification up to about ISO speed 400 or 800.
I can't agree that none of the recent Nikon DSLRs are completely ISO-less in practical terms. If you check out the results for the D7000 at DXOMark and compare noise levels at ISO 100 and ISO 3200, which represents a 5 stop difference, you'll find that the differences in DR measurements between ISO 100 and ISO 3200 is exactly 5 EV, or 5 stops. If I've interpreted this correctly, it means that a shot with an underexposure of 5 stops at ISO 100 will have exactly the same degree of shadow noise as that same exposure used at ISO 3200.
As regards SNR at 18% grey, the results are very close. At ISO 100 we have 41.1 dB. At ISO 3200, 26.7 dB. That's a difference of 14.4 dB. On the basis that 3 dB is equivalent to a difference of 1 EV or 1 stop, an underexposure of 5 stops at ISO 100 should result in a fall of 15 dB in SNR. One gains a mere 0.6 dB in SNR at 18% by using ISO 3200. Wow! I bet you that difference would be invisible on any size print, even on the creamiest skin of the most beautiful model.
The DXO results for the Nikon D800E are similar, with the exception of DR between ISO 100 and 200. It's less than it should be for an ISO-less camera. However, SNR at 18% is exactly 8 stops down
at ISO 25,600, or 24 dB down, which is what 8 stops of underexposure at ISO 100 should produce. The D800E can be described as a true ISO-less camera from ISO 200 to ISO 25,600. The loss in DR in those 7 stops of underexposure is a mere 1/3rd of a stop, and there's no loss at all in SNR at 18%. That's not bad.
If we compare the DXO measurements for the D7100, we find that the ISO-less nature is not quite as good as that of the D7000 and is best within a narrower range of ISOs, specifically from ISO 200 to ISO 1600, which represents the 3 stops underexposure that I used in my above shot of the mule.
According to DXOmark, I've sacrificed 1/3rd of a stop of DR, and 0.3 dB of SNR at 18%, by underexposing 3 stops at ISO 200 instead of raising ISO to 1600. DXOMark in their articles on such matters claim that a difference of 0.5 EV in DR should be noticeable, implying that any differences less than 0.5 EV might not be noticeable, or not significant.
In summary, the D7000 is very close to the true ISO-less camera, closely followed by the D800E. The D7100 is clearly a backward step in this regard, particularly in view of the banding which is not apparent in either the D7000 or D800E.