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Old 04-06-2013, 05:06 PM
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rmuhlack (Richard)
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Using ISO settings greater than unity ISO value - why...?

I have been reading Roger Clark's article on sensor performance, in particular his comments on "unity gain" (here).

His statement that "Since 1 electron (1 converted photon) is the smallest quantum that makes sense to digitize, there is little point in increasing ISO above the Unity Gain ISO (small gains may be realized due to quantization effects, but as ISO is increased, dynamic range decreases)." has got me thinking about the super high ISO values that people often mention when taking nightscapes.

If Roger's statement is correct, what is the benefit (if any) to using an ISO value of say 3200, 6400 or even 12800 if the unity gain value is much much lower?? Is it simply that the image doesn't need to be stretched as much in processing, or is there more to it than that? A genuine question, as I am tossing up whether I need to upgrade my camera body sooner rather than later, or if I can spend the dosh on new lenses instead.

My current daytime camera (a 400D) has an ISO range of 100-1600 (or up to 3000 if i use a firmware hack), however the unity gain ISO is ~1220. So where am I losing out by shooting at ISO1600 in comparison to a new camera (say a 5DII) which has a maximum ISO of i think 25600 yet has a unity gain ISO of about 400??

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Old 04-06-2013, 05:52 PM
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Perhaps a similar concept as I am not sure Roger defines how to calculate unity gain.

DXOMark rates sensors to defined criteria. One of those is highest ISO without a loss of image quality. Per them 5D2 ISO is 1815 not 400. If Roger is saying its 400 there must be an error as there are many many fine 5D2 images taken at ISO3200


My understanding is when you increase ISO you reduce dynamic range. For example a lot of cameras maximum dynamic range is at its lowest ISO.

So if you shoot above this DXO Mark ISO you will get reduced dynamic range as the main consequence (blown highlights usually).

Theoretically you would get the same image using ISO6400 as at ISO1815 and then boosting the exposure compensation in software.

But I believe this unity gain concept is not 100% correct in that modern cameras also have clever noise reduction algorithims so you get better performance than you would have with earlier models at higher ISO than the rating may suggest.

If you were doing a tracked image where star elongation isn't the limiting factor to the exposure time (like it is with untracked images) then I would use the closest ISO to this DXOMark rating and make up for the lack of brightness with a longer exposure.

Again lens considerations complicate this further. A bright F2.8 or faster widefield lens with little or no aberrations (coma, distortions, chromatic aberration ) further enable you to get a bright image with minimal star elongation at a reasonable ISO.

ISO is merely an amplifier of the voltage before analogue to digital conversion. Like turning up the volume on your stereo. Eventually it gets distorted.

Last edited by gregbradley; 04-06-2013 at 06:13 PM.
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Old 05-06-2013, 06:30 PM
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Shiraz (Ray)
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good question Richard

+1 Greg's comments and, FWIW, add the following:

All ISO setting does for astro is complicate matters by making it seem as though you can make the camera detect dimmer targets by setting the ISO higher. As you point out though, you could get the same result by scaling the signal in an image taken at lower ISO. Provided you have the gain (ISO) set to ensure that the signal on extremely bright targets is just below the RAW saturation level, the dynamic range will be as wide as possible and the SNR will also be as high as possible. Higher ISO does not provide any advantage at all, since you do not need to manipulate depth of field, freeze image motion etc - plus you start to lose dynamic range. Its a pity that cameras do not have a "science" setting which sets the gain to optimise the dynamic range and turns off all pre-processing - shouldn't be difficult.

Just a guess, but I suspect that the nightscape folks use high ISO so they can see what is going on - no point in taking a sequence and only finding during later processing that the focus, depth of field or framing is not what you wanted.

In case you haven't already seen it, this is worth a very careful read http://www.cloudynights.com/item.php?item_id=2786

regards Ray

Last edited by Shiraz; 06-06-2013 at 10:01 PM.
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