This answer to the question about how ISO is implemented in digital cameras seems to imply that each photosite (i.e., pixel) can have its ISO set independently. If this is true, then I would think that it is theoretically possible to take a photograph in which certain photosites are at a different ISO than others. The first part of my question is: Assuming variable ISO is possible, would it be useful? It seems to me that this might be a useful way to increase the dynamic range of the sensor, e.g., by choosing a high ISO only for regions of the image that are in the shadow. Assuming variable ISO would be useful, why hasn't it been implemented in digital cameras yet? (Or has it?)
Answer
The closest thing I know to what you're thinking of is what Fujifilm are doing with DR mode in their EXR sensors, as seen in the X-10 and X-S1) - half the pixels are deliberately underexposed by a stop (or two) and combined with the "normally" exposed pixels before the image is output. For more detail, see DPReview's X-10 review - what you're interested in here is the 6 MP DR mode, rather than the 12 MP DR mode, which is the standard "underexpose and then apply a different tone curve to the whole image" seen in many cameras these days and trades off shadow noise for increased dynamic range. The 6 MP DR mode is interesting as it (in theory) allows you to increase dynamic range while keeping shadow noise as it would normally, although of course you're paying the cost in resolution instead.
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