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Author Topic: ISO levels  (Read 3670 times)

coreyhkh

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ISO levels
« on: August 19, 2012, 07:15:32 PM »
Since getting my 7d I have noticed it has more iso steps them my K5, with the K5 is 100,200,400 and so on but the 7d has far more levels. I have seen a lot of people say don't use the half stops why is this? does using the odd numbers increase noise?

Thanks
« Last Edit: August 19, 2012, 07:28:56 PM by coreyhkh »

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ISO levels
« on: August 19, 2012, 07:15:32 PM »

Mt Spokane Photography

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Re: ISO levels
« Reply #1 on: August 19, 2012, 07:56:50 PM »
You can change the behavior in setup.
I'm not sure that there is any absolute proof that full stop values produce less noise, but some have claimed that their tests show this.

neuroanatomist

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Re: ISO levels
« Reply #2 on: August 19, 2012, 08:27:14 PM »
The 'tweener' stops are pushed or pulled from full stops.  Pulling is ok (a little better than ok, actually, there slightly less noise than the next-higher full stop), but pushing increases noise. So, ISO 125, 250, 500, 1000, etc. will have a little bit more noise.  In the C.Fn's you can set it so ISO is selected in 1/3 (default) or full stop increments.  If you use Auto ISO, the camera will use 1/3 stops regardless of that setting.

Personally, I have it set to full stops - but that's not because of the push/pull noise effects (which I believe are real, but practically not significant), but rather because it means I can ramp ISO up or down 3x faster with fewer clicks of the wheel - and that's an actual practical benefit.
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Ryan708

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Re: ISO levels
« Reply #3 on: August 19, 2012, 10:21:31 PM »
Interesting, I've not heard of this before. Might have to try and see if I can tell difference.
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coreyhkh

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Re: ISO levels
« Reply #4 on: August 19, 2012, 11:29:23 PM »
Thanks for the info.

DArora

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Re: ISO levels
« Reply #5 on: August 20, 2012, 09:46:57 PM »
The 'tweener' stops are pushed or pulled from full stops.  Pulling is ok (a little better than ok, actually, there slightly less noise than the next-higher full stop), but pushing increases noise. So, ISO 125, 250, 500, 1000, etc. will have a little bit more noise.  In the C.Fn's you can set it so ISO is selected in 1/3 (default) or full stop increments.  If you use Auto ISO, the camera will use 1/3 stops regardless of that setting.

Personally, I have it set to full stops - but that's not because of the push/pull noise effects (which I believe are real, but practically not significant), but rather because it means I can ramp ISO up or down 3x faster with fewer clicks of the wheel - and that's an actual practical benefit.

Thanks for the information neuro. I have couple of questions though. How do you figure out which one is pushed and which one is pulled ISO? My guess is that 2/3 stop should be pulled from next full stop and 1/3 should be pushed from previous full stop? Is that correct? In that case will it be better to use ISO 1250 than ISO 1000?
Also how does this push/pull affect ISO 50?

neuroanatomist

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Re: ISO levels
« Reply #6 on: August 21, 2012, 03:16:35 AM »
Correct - ISO 1250 is pulled from 1600, 1000 is pushed from 800, etc.

ISO 50 is an expansion setting (L), and like the H settings, means digital gain (negative for ISO 50). So, basically pulled a stop from ISO 100.
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Re: ISO levels
« Reply #6 on: August 21, 2012, 03:16:35 AM »

marekjoz

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Re: ISO levels
« Reply #7 on: August 21, 2012, 04:21:59 AM »
It is really helpful to use pulled ISOs with video as there is not an easy option to improve (and not make even worse) it in postproduction if you used the full stop but it showed to be to dark/to bright on video.
With stills if you shoot RAWs it has less meaning. If you shoot JPGs, then it's better to use: full stops or pulled ISOs by 1/3 stop.
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DArora

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Re: ISO levels
« Reply #8 on: August 21, 2012, 02:09:39 PM »
Thanks neuro & marek!!  :)

hyles

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Re: ISO levels
« Reply #9 on: August 21, 2012, 03:43:35 PM »
I thought that all ISOs value where pushed from 100 ISO.
Diego

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Re: ISO levels
« Reply #10 on: August 21, 2012, 03:49:34 PM »
I thought that all ISOs value where pushed from 100 ISO.
Diego

I think, that if it was the case then achieving ie 8DR at 3200 would mean ie 20DR at 100. :-)
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neuroanatomist

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Re: ISO levels
« Reply #11 on: August 21, 2012, 04:22:51 PM »
I thought that all ISOs value where pushed from 100 ISO.

Sort of. But there's analog gain prior to the conversion to digital (which is what 'native ISO range' means), and there's digital gain after ADC (expanded ISO settings).  I believe that 'tweener' ISOs are also digital manipulations of analog gain applied in full-stop increments.
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Studio1930

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Re: ISO levels
« Reply #12 on: August 21, 2012, 04:40:01 PM »
I thought that all ISOs value where pushed from 100 ISO.

Sort of. But there's analog gain prior to the conversion to digital (which is what 'native ISO range' means), and there's digital gain after ADC (expanded ISO settings).  I believe that 'tweener' ISOs are also digital manipulations of analog gain applied in full-stop increments.

I have wondered about this as well.  So are you saying that full stops (100, 200, 400...) are achieved by an analog amplifier but the in between ISOs (125, 160, 250...) are achieved by an analog amplifier and then multiplied or divided by a factor digitally which is what reduces the dynamic range?   ???

This chart is not showing that full ISO has more dynamic range than in between ISO except for the 5DM3 and they don't match full ISO increments.

http://home.comcast.net/~NikonD70/Charts/PDR.htm#EOS%201D%20Mark%20IV,EOS%205D%20Mark%20III,EOS%201D%20X
« Last Edit: August 21, 2012, 04:55:43 PM by Studio1930 »
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Re: ISO levels
« Reply #12 on: August 21, 2012, 04:40:01 PM »

marekjoz

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Re: ISO levels
« Reply #13 on: August 21, 2012, 05:17:49 PM »
I thought that all ISOs value where pushed from 100 ISO.

Sort of. But there's analog gain prior to the conversion to digital (which is what 'native ISO range' means), and there's digital gain after ADC (expanded ISO settings).  I believe that 'tweener' ISOs are also digital manipulations of analog gain applied in full-stop increments.

This is how it is easier to understand (in reality it is more complex)
1. Signal from sensor needs to be amplified because it is too weak to go straight to the A/D converter. There is a base "best" amplification factor at which the signal achieved after amplification will have the best signal to noise ratio preserving still the usable information. This would be the amplification (gain) required to get base ISO - let's say 100.
2. The amplified signal goes to the A/D converter which change analog electric signal (it's voltage) to the digital domain (numbers). They work simple - if you have an electric signal from 0V to (let's say max) 2V and you assume, that 0V is black and 2V is white (in terms of luminocity) and you want to get 16 bits resolution then you have 1 of 65536 different values representing the electric signal in the moment of sampling. Of course you need to have luminocity of pixels of different colors (RGB or even RGGB to get a color value). So in terms of image it would be luminocity value (shade of grey) of a single pixel. Imagine that electric signal from sensor is constructed by getting values from each pixel one by one.
3. If the signal is too weak it needs to be amplified stronger (like getting louder music). If you don't amplify it, then after A/D conversion you could get numbers only from the range let's say 0 to 10000. By amplifying weak signal you loose some information and introduce additional noise. There is a noise in the unamplified signal so by amplifying the signal you also make the noise more relevant. This is where dynamic range falls down and noise rises up.
4. Amplified and digitized signal represents the color values of the sensor's matrix. Digitized signal means that it's represented by values - from 0 to 65535 (in 16 bit resolution domain). If we have 14 bits resolution domain, then we have 16384 possible values for each pixel (in reality multiplied by three as there are 3 colors but here he talk about luminocity value of a single color value).
5. Let's say, that amplification gain required to achieve ISO 100 is equal to 5. Let's say, that gain required to achieve ISO 200 is equal 10 (this is just for this example). So it means, that if you could amplify the analog signal (before digital conversion) by 7.5, it would be equal to ISO 150.
6. But if you first get digital numbers at ISO 100 and later after A/D conversion multiply the achieved values by 1.5, you will get digital values "as if" they would be achieved with the analog gain equal to 7.5. This would be "pushing" ISO.
7. Same if you would divide digital values achieved at ISO 200 by ...(guess number :) ) you would get almost similar "values" like before - by applying analog gain equal 7.5 and it would be "pulling" ISO.
8. Why is it bad? Because in the amplified analog signal so also in converted signal, there can be values, which after the digital arithmetics would go beyond the scale (you cut off highlights so the become simply white) or they become more compressed at lower scale areas (so you compress shades and get banding) Why banding? Because if you have values 10 and 11 representing luminocity of pixels in a row you don't see banding. But if you divide 10 by 2 you get 5. If you divide 11 by 2 you get 5.5 which rounded down also gives you 5. This two pixels after digital arithmetics become the same.

Well... Short answers are better. I think I have described this quite good enough for simple imagination. In reality there are more tricks and it doesn't look quite like described above. I hope it helps to understand the process.

Edit: changed D/A to A/D :-)
« Last Edit: August 21, 2012, 06:38:42 PM by marekjoz »
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Studio1930

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Re: ISO levels
« Reply #14 on: August 21, 2012, 06:29:48 PM »
The above post is a good explanation of how it works, but I don't think it addresses the question of whether or not the in between ISO ranges are analog boosts or digital multipliers.  I think it is probably dependent on the camera in question (meaning that some of them do it differently).
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Re: ISO levels
« Reply #14 on: August 21, 2012, 06:29:48 PM »