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Old May 18, 2008, 9:23 AM   #1
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Can someone please explain native ISO and what it means noise wise? I read on a few forums people being told to stick with ISO 100 200 400 800 etc rather than 125 320 650 etc. Apparently there is more noise in 320 than 400. I don't get it. Is this just a fallacy or what? I often choose to use ISO 640 over 800 and now I'm wondering if that is a good Idea
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Old May 19, 2008, 1:51 AM   #2
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Hi AJ,

I can't dig up any links at the moment, but when I looked into it last I was satisfied that there WAS more noise at the intermediate settings.

I seem to remember though that it was more like the noise at every intermediate step was the same as the higher "full" ISO setting.

e.g. ISO 125 = ISO 400.

This also mainly has an impact on JPG shooting where it seems that they might just have been cheating. i.e. You set to ISO 125, they set in camera to 400 and then underexpose by 3/4 stop, which of course can lead to some horrible effects at the low and high ends of the histogram, and is wrongly interpreted as high-ISO noise, whereas it is in fact underexposure noise which is often worse. (See the LL expose to the right article for why this would be the case).

In any event I have found that the Canon RAW files can easily be adjusted by 1 stop in either direction if you get your exposure right.

So I always stick to the native ISO settings.
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Old May 19, 2008, 5:00 PM   #3
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MMM Im not sure that ISO 640 looks as bad a ISO 800, but like you said, I shoot RAW so maybe that is the difference
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Old May 19, 2008, 5:50 PM   #4
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I can remember reading some threads a while back elsewhere discussing this issue.

From what I understand about it, the normal amplification of the signal when you increase ISO speed is done prior to the analog to digital conversion (with the exception of the "boost" or "extended" settings above ISO 1600).

But, with some of the Canon models, apparently (based on threads I've seen from people that seen to know more about it), they're just "pushing" the exposure after the analog to digital conversion when you use the "in between" ISO settings (basically, multiplying the values to increase the image brightness). Hence you sometimes see the term "fake" used when describing these in between settings.

This is the same technique normally used by manufacturers for ISO speeds above 1600. For example, with most dSLR models, ISO 3200 is just underexposing the image a stop from ISO 1600, then increasing the values from each photosite after the A/D conversion to simulate a higher ISO speed).

Even if you do shoot raw, it's a good idea to try and expose a bit brighter when possible (since you're just 'pushing" the exposure in the same way if you need to move the exposure compensation slider to make an image brighter, which tends to hurt dynamic range and increase noise). The only major difference is that you're doing it versus the camera doing it.

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Old May 20, 2008, 5:13 AM   #5
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JimC wrote:
Quote:

Even if you do shoot raw, it's a good idea to try and expose a bit brighter when possible (since you're just 'pushing" the exposure in the same way if you need to move the exposure compensation slider to make an image brighter, which tends to hurt dynamic range and increase noise). The only major difference is that you're doing it versus the camera doing it.
but would that cause blow outs ?
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Old May 20, 2008, 5:30 AM   #6
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Only if you shoot so bright you're losing detail in highlights you want to keep. IOW, try not to underexpose the image unless you need to.
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Old May 20, 2008, 7:09 AM   #7
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P.S.

See the article that peripatetic referred to if you haven't read it yet:

http://www.luminous-landscape.com/tu...se-right.shtml



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Old May 21, 2008, 10:33 PM   #8
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I don't get it... According to dpreview noise graphs (for both luminance and chroma) the noise levels are pretty linear with increasing ISO (for the 40D):





-> If the intermediate steps were to be more noisy wouldn't the 40D gragh looks like this orange curve from the D80 (i.e. 500, 640 as being more noisy than ISO 800)?





Also isn't the native ISO pretty arbitrary depending on the camera's sensor? Unless someone has actually test this how can one determine that one particular ISO value is native while the others are computed or pushed?
-> i.e. may be the 640 is the sensor native ISO and 800 is computed...
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Old May 21, 2008, 11:46 PM   #9
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I dont get it either, I do know that ISO 800 is more noisy on my 30D than ISO 640 though
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Old May 22, 2008, 2:42 AM   #10
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Yeah, but I wouldn't put too much stock in those dpreview graphs. Clearly they are straight lines between the full-stop values.

I have no direct experience of this stuff, because I just always shoot at full ISO stops anyway.

There is no a-priori reason why the sensor shouldn't be settable at continuously variable gain.

I have read somewhere that the 5D is now the only camera in Canon's DSLR lineup using an OS used under licence by Canon. All their other cameras now use a Canon-owned system.

Perhaps this stuff only applied to the 20D/30D era of cameras.

At any rate if AJ sees less noise in ISO640 than ISO800 I guess that's QED.
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