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I don't know exactly how ISO works within a digital camera, but from a video I saw, it described it as the camera essentially brightening the image from the sensor, given the same amount of light from the shutter/aperture.

If that is so, I don't see why a camera couldn't record two or more ISO's at the same time and record them as different images. Why is this not possible? Is changing the ISO sensitivity more tied to the mechanics of the sensor/camera so this is not possible? Or is it possible but just not done because there is no good reason to implement it?

I'm asking because if this is possible, it would make ISO variable HDR photos a lot easier with no ghosting and open up possibilities in many cases where it is not used right now (fast moving objects).

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    Possible duplicate of Variable ISO Sensor: Possible and/or Useful? – mattdm Apr 30 '17 at 7:49
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    I think that one is asking if different pixels can read out different ISO values (different ISO values for different pixels). I'm asking if two images can be taken at the same time (so the whole sensor reads out two images with different ISO's at the same time). – James Verdune Apr 30 '17 at 8:11
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    Possible duplicate of Could a "universal exposure" setting be practically possible? – scottbb Apr 30 '17 at 12:11
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    @mattdm - that question is blatantly not what the OP is asking. You are too keen to flag duplicates. – Myridium Apr 30 '17 at 14:37
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    I'm pretty sure the second one is, with the clarification. And I think the first one certainly could be. I don't think anything is "blatent". I also don't think I am "too keen" — I think it's useful to show people existing information on the site. We can take this up on Meta if you like. – mattdm Apr 30 '17 at 14:40
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Yes, it's entirely possible, and it has in fact been done with Magic Lantern's "dual-ISO" mode. But you can't get two images simultaneously. What you get are two half-images.

This leads to drawbacks. The way it's implemented in ML's dual-ISO mode is that alternating pairs of scanlines use different ISO settings. The problem is that you then have to interpolate the data to get a usable image, so you are going to have a loss of resolution in highlights/shadows, and aliasing and moire issues.

But, yes, this method does give you single-shot HDR without any ghosting/clone issues, and makes HDR video possible.

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If that is so, I don't see why a camera couldn't record two or more ISO's at the same time and record them as different images.

Why would one want to? The raw voltages off the sensor in each case would be the same. The only difference would be the amount of amplification applied to the signal when it is digitized. Multiplying/dividing the digitized values are what we already do when post-processing raw files. In a very real sense, buy saving the raw data and processing the image with varying settings we can already do what you suggest - just not in the camera at the time the photo is exposed. Of course the more the signal is amplified when converted from analog to digital, the lower the dynamic range will be.

Having the same information recorded at two different amplification levels would not increase the dynamic range of the sensor - that is determined by the distance between the noise floor and full well capacity. But multiplying it by a greater factor can reduce the dynamic range by reducing the maximum amount of light recorded before the value for full saturation of a pixel is reached.

It's not like you could shift the shutter time or aperture value based on the varying amplification. Those would be locked in for each single exposure.

I'm asking because if this is possible, it would make ISO variable HDR photos a lot easier with no ghosting and open up possibilities in many cases where it is not used right now (fast moving objects).

You would still be limited by the sensor's physical characteristics. The difference between the minimum signal that can be distinguished from noise and the maximum signal that can be distinguished from an even higher signal is what determines the sensor's dynamic range. Assuming one is recording the raw data then it is all available to work with when editing the image.

If one is only wishing to save as a jpeg, there are already cameras that can make multiple jpegs from the raw sensor data of a single exposure and alter the the in-camera processing to produce multiple jpegs. This is usually done in terms of white balance bracketing, but it can also be done with regard to amplification/image brightness in a few cameras. I suppose it would be trivial to also do it while varying contrast as well.

  • I think there could be a reason. I copy hat you are limitted by the physical characteristics of a sensor. Another limit is quantization hwen the signal is digitized. Say we have 14 bit depth, that means we have 2^14 levels of the brightness for whole the dynamic range of the sensor. If I use simultaneously two ISO levels, say 100 and 200, the figure shot with ISO 200 will overflow the INT14 values but I will have twice more levels in the shades. All that applies if the ISO amplification is done prior/during the digitization. If it is postprocessed, there is no advantage. – Crowley May 2 '17 at 19:07
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You mean using Correlated Double Sampling (CDS) like a Sensor from the company Cmosis, using two ADCs per pixel or a Dual-Gain ADC - yes, it's possible.

It's usually expensive to get extreme HDR but there are relatively inexpensive Security Cameras that can see faces in an underground Carpark and a clear background in the bright Sun.

Try searching for more info.

Here's something to refer to: https://books.google.ca/books?id=Vp2LBgAAQBAJ&pg=PA171&lpg=PA171&dq=two+adcs+per+pixel&source=bl&ots=2veAdcfp3R&sig=-HgNv2DgdEUkAe1pUG7_41cUBtQ&hl=en&sa=X&ved=0ahUKEwjw8Izin53UAhWGxFQKHcEFDxwQ6AEIQzAI#v=onepage&q=two%20adcs%20per%20pixel&f=false .

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