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[EDIT: The original main question here was "What is Highlight Tone Priority?" It turns out that Highlight Tone Priority, Active D-Lighting, D+, and Adaptive Dynamic Range all mean the same thing, so that was an exact dupe of this question.]

The non-duplicated part of the original question was: why is ADR incompatible with ISO expansion?

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  • The same thing (although perhaps a less technical version of): photo.stackexchange.com/questions/2005/… If you understand my answer, you can extrapolate why it does not work with ISO expansion on the lower end.
    – eruditass
    Aug 5 '10 at 14:52
  • So Adaptive Dynamic Range is Nikon-ese for the same thing? I'm not sure I get why you can't have ISO expansion as well, though, particularly if HTP/ADR are only applied to the JPG and not the original RAW file...?
    – Matt Bishop
    Aug 5 '10 at 15:03
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    Nikon calls it D-Lighting, ADR is a generic term. Your whole RAW file will be underexposed by 1 stop. If the minimum ISO with expansion is 100 ISO, this underexposure means that for the whole image (except the highlights), in order to get it to the correct exposure, you need to add +1 EV in post, making the image ISO 200. This is why if you underexpose images, you will get more noise than expected because you add + EV and basically need to increase the sensitivity to get it to look correct.
    – eruditass
    Aug 5 '10 at 15:19
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    I think I get it - ISO 100 isn't available because the camera would have to actually take the shot at ISO 50, which it doesn't support. And I'd guess that ISO 6400 isn't available because it would actually be shot at ISO 3200 plus 1EV in post, which would be unusably noisy?
    – Matt Bishop
    Aug 5 '10 at 15:40
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    Yep, you got it down!
    – eruditass
    Aug 5 '10 at 17:42
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Highlight Tone Priority (HTP) changes the exposure settings used by the camera to ensure that the highlights are not over-exposed. This is especially helpful in strong daylight pictures with lots of detail in the highlights. A great example of a time to use it is a daytime wedding in the sun, when you are trying to get the details in the white wedding gown.

The downside of using HTP is that the picture will basically just be under-exposed. This is perfectly fine if your image is generally high key and low contrast, but if you have a high contrast image, then your shadows will be quite under-exposed, which will increase the noise.

I believe the reason that ISO expansion would be disabled is that the ISO is internally modified down a bit to lower the overall exposure.

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