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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?)

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Sounds technically possible but may require too much circuitry to do at pixel-precision and may be hard to scale and cause too much heat. Moreover, it is not clear this works better than current solutions such as reading photosites partway during the exposure or having photosites of difference sizes, giving them different native sensitivities. – Itai Sep 9 '12 at 19:03
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There's a little catch-22: you need to set ISO before reading a pixel value, but you'll know the pixel belongs to a shadow area only after reading the value. – Imre Sep 10 '12 at 21:07
@Imre True, but that's not necessarily a technical problem. For example, as Itai mentioned above, there is already technology to read photosite values partway through the exposure. Advanced metering systems could also be used to "guess" ISO values for regions. Finally, for still shots like landscapes, an initial test exposure could be used to set the ISO values for a second shot. – ESultanik Sep 11 '12 at 12:28
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It should be noted that ISO does not change anything about what the sensor or pixel is actually capable of. The only thing the ISO setting does is change the white point of a given exposure. Sensors are fixed linear devices that are capable of registering a fixed charge (count of electrons) in each pixel, +/- the average of electronic noise (which these days on a normalized basis is only a few electrons.) By increasing ISO, all your doing is saying that instead of "white" being achieved at 40,000 electrons, its achieved at 20,000, or 10,000, etc. – jrista♦ Sep 13 '12 at 21:54
What occurs at each pixel is row/column activate and charge readout. During readout, that charge is amplified by the necessary amount to "saturate" according to the ISO setting, and at the same time, a variety of electronic noise compensation may be applied as well (in the D800, there is a bunch of circuitry dedicated to mitigating electronic noise, which is why its lowISO DR is so good.) Logically, I don't think such a thing as variable ISO would apply. The solution to low-SNR noise is to reduce electronic noise...and Sony has achieved that in their Exmor sensors. – jrista♦ Sep 13 '12 at 21:56
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3 Answers

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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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Essentially, a sensor like this that would have variable exposures for each photo site would have an image that needs to be tonemapped during the RAW conversion process. More information would have to be sent with each pixel, and this would increase the size of the transmitted data, along with the processing power required in the camera. That's a mere technical issue, and I'm sure that in another few years, that won't be a problem at all.

The biggest headache I see would be making sure the popular RAW conversion programs would support the decoding process. The resulting RAW file might need to contain 32-bit color information, and there is very limited support for operating on 32-bit color images today. For the most part, they need to be tonemapped down to 16-bit first. This isn't a process that will yield great results if done automatically with today's software.

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Really I do not see manufacturers caring about such a headache. That's why they have proprietary RAW format and Fuji never stopped at creating odd arrangements of pixels with various sizes and color filters. If they can get an edge from it, I expect them to do it. Most high-end image processing applications, including Lightroom & Bibble (AferShot now), work in 32-bits internally already. Its more efficient to work in 32-bits linearly with modern processors. The first paragraph you wrote makes sense to me though. – Itai Sep 9 '12 at 20:57
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CMOS sensors are already basically an array of sensors with different ISO, which they have to compensate for. This is what gives the plastically look on CMOS sensors, but also what attenuates blooming.

However, they actually already do make CMOS chips with multiple "ISO" for achieving a higher dynamic range, where the pixel size area is double for half of the pixels, or one out of the two green pixels are twice as sensitive as the other. The cost is more transistors per pixel, which can give trouble with noise and overall sensitivity, due to leaving less space for the photo sensors. Large pixel light-integrating cells result in lower noise (generally), which is why a 36x24mm sensor at X Mpixel is better than a 1/3 inch sensor at X MPixel - they respond better to the light to overcome the noise from all the electronics.

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