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09-29-2009, 12:32 PM   #1
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How do expanded ISO ranges work?

For many cameras, expanded ISO is offered. For cameras that the base ISO is 200, and the analog amplifier only goes up to 3200, they expand it to 100 - 6400. I understand how to get 6400, you under expose and push it digitally - but now do you get iso100 or less? Certainly you can overexpose and pull it down - but how do you save the highlights?

When I put on that hacked firmware on my SO's SD1100IS, it offered ISOs as low as like 16. I never tried it, but if it worked (without overexposing, blowing highlights, and pulling it down), depending on the quality, it would certainly seem useful to get long exposures in daylight (like to get flowing water) while keeping the aperture above the diffraction limit - without using ND filters.

So how is it done? Can we do it ourselves?

09-29-2009, 09:52 PM   #2
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ISO ratings below the base Native ISO are done by digital processing.
You won't actually get blown highlights, because the exposure system will take care of that.

The drawback is you lose dynamic range. For every stop of ISO you go below the Native ISO, you lose 1 stop of dynamic range at the top end.

So they are not really a good substitute for ND filters - unfortunately the "pretend" extra low ISO shots have that flat, low-contrast look.

Canon did a write up for their users on the Expanded ISO function

Last edited by kittykat46; 09-29-2009 at 09:58 PM.
09-29-2009, 10:52 PM   #3
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I still don't get it. Walk me through it.

I have a shot that is 1/10 seconds long, f/1.4, and ISO 100. I create the fake ISO50, so naturally either 1/10 goes to 1/5, or f1.4 goes to f2.0. Then digitally you go in and dial down the exposure / adding less than 1 gain. However, the sensor still collects data at ISO100 - blowing highlights.

Well, I guess I had it write in the original post - overexpose and pull it down - and you don't save the highlights, you lose 1 stop in the highlights.

Ok, makes sense.
09-30-2009, 02:54 AM   #4
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I think it is not necessary always done digitally. It just shows that camera is being operated out of the optimal ISO range, so something most likely is being sacrificed.
At high ISO's this might just be the level of reasonable quality, where extended mode clearly states that the camera is being pushed over its limits and you shouldn't expect anything good.
Maybe normal iso is just the range at which metering produces most useful results and you wont get weird Av/Tv combinations, like 1/2000@F4 in daylight.

Instead of increasing signal amplification, you could, for example decrease the reference level of ADC (analogue value which equals full digital range, like 1V=1024 or 0.5V=1024). In such case much lover analogue value per bit had to be converted, taking greater impact from conversion circuitry.
At the low site it could be not so good reference level. Because if you can get stable and clean 3V from 6V battery, you may not have enough headroom to get stable 5V reference if card writes, flash charges can AF current gulps can cause battery voltage drops to 4 volts.
Most likely the programmable amplifier has free enough choice of amplification, but again when inputting extreme values you may not know exactly when it's hitting power supply limitations or getting non-linear.

So with this i'm saying that this may not always be a digitally pushed mode, there are many other options that can be used and get out of their qualitative ranges.You cannot tell it for sure unless you do a qualitative, controlled test with a pre-bayer,pre-dark RAW file.

Last edited by ytterbium; 09-30-2009 at 03:01 AM.

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