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Old 29-06-2010, 10:28 PM
bloodhound31
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Higher ISO more or less noisy?

Hi folks, my thread asking for wide lenses without curvature has turned into a very interesting off-topic conversation about ISO, and whether it is more and less noisy as you increase it.

I thought it deserved a thread of it's own as there is some very good information coming out of it, worthy of discussion among the more knowledgeable members of IIS photographers, both terrestrial and astro.

It started here, and developed.

Please continue on this new thread here to make it more searchable by topic heading. It's great stuff!

Baz.
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Old 29-06-2010, 10:51 PM
luigi
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There were some incorrect comments on the former thread so I will continue here.

First of all two keys:

To minimize noise you always want to expose longer, when you can't expose longer if the histogram still allows it then increase the ISO. Higher ISOs have better signal to noise ratio than lower ones.

A 1 second exposure at ISO800 has LESS noise than a 1 second exposure at ISO100.

ISO is a hardware amplification of the signal that the camera sensor reads. We can divide sources of noise in two categories:

1) Noise that is amplified by the ISO gain (Npre)
This includes:
- Photon fluctuation noise
- Read noise (sensel read error)
- Thermal Noise

2) Noise that occurs after the ISO amplification (Npost)
This includes
- Analog to digital conversion error
- Rounding to fixed point

So we can formulate the signal as:

[(So +/- Npre) * ISO] +/- Npost

In this formula we can see that Npost is constant and doesn't depend on the ISO amplification therefore as the ISO gets higher the Signal to Noise Ratio improves.

It is true that as exposures get longer Npre starts to dominate over Npost but it is also true that a higher ISO can't have a worst S2N ratio than a lower one so you should also use the highest ISO you can afford for the longest exposure you can afford.

This holds only for native ISOs, ISOs that use hardware amplification. In my camera those are ISO 100,200,400,800,1600 and 3200. Intermediate ISOs and expansions (fake) are not useful because they use just a software amplification and therefore amplify both Npre & Npost noise.


This paper from Dr Emil Martinec is a good reference:
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/

This graph shows it nicely:
http://theory.uchicago.edu/~ejm/pix/...Rwindow1d3.png

Luis
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Old 30-06-2010, 08:33 PM
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philiphart (Phil Hart)
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if you keep searching, there are a number of other older threads on this topic. here's an old test i did that is still relevant for canon cameras:

http://philhart.com/content/astropho...xposure-length

i'm convinced that higher ISOs are the best option (for the canon cameras i have used) until your sub-exposures are long enough that you are burning out bright areas, when you can start to dial down the ISO a little. for short subs, ISO1600 is best, but you can hardly tell any difference with 800 too. Higher than ISO1600 makes no difference as it is only a software gain rather than anything in hardware so signal to noise stays the same.

this pic was taken with 3 minute ISO1600 subs. if you know of a better SLR shot of this region, then I want to see it!
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Old 30-06-2010, 08:56 PM
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It certainly is a very nice picture, Phil!
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Old 02-07-2010, 02:44 AM
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JD2439975 (Justin)
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In the not too distant past I did an ISO-exposure test on my 40D, the full details of which can be found Here.

My reasoning was that as I take several hours of lights, then darks, other object lights & more darks, the best I could hope for was some before (warmer) & some after (cooler) darks that didn't exactly match my lights.
My thoughts were to minimise the noise in the first place so any difference was also minimised, but I digress...

These values are "rough & ready" so should be taken with a grain of salt, especially in the low ISO region.

Below is the mosaic of dark frames & the table of average values, this seems to agree that for a given signal/exposure SNR will increase with increasing ISO, at least for thermal noise.

Attachment 73517 mosaic.

Attachment 73519 average values.

If we take for example 1min exp @ISO800 given a signal value of 16 & noise at 16 this would give SNR of 1:1.
At ISO1600 signal is amplified to 32 with noise at 22, or SNR of 1.45:1 & finally for ISO3200 SNR increases to 2.2:1.

That's all well & good but in my meager experience 5 min subs at ISO1600 or 3200 have horrendous noise even after dark subtraction (uncooled, tropical temps remember).

So looking at stacking an equivalent, same exposure or true signal (No of photons)...
2x 1min, ISO1600 - SNR 32/22 or 1.45:1 each frame.
1x 2min,ISO800 - SNR 32/18 or 1.78:1.

Ignoring any dark subtraction & I know this is a very simple case of only two frames but an "average" stack would not change the SNR at all in the ISO1600 case.
Even further if the signal was faint (as many astro sources are) & only supplied one photon per 2 min interval then the SNR of the average stacked 1600's would be halved as only one would contain signal.

This is an over simplified & extreme example but you can see where I'm going with it...

I'll agree that for a single, given length exposure that high ISO has greater SNR & that using as high as you can for a given exposure is good but for faint, stacked astro work...lower & longer still gets my vote.

Feel free to set me straight on this, always willing to learn something new in this area.

Justin.
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Old 02-07-2010, 04:08 AM
luigi
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Justin,
Longer is always better.
When you can't expose longer you should use the highest ISO available.
I think I've already said this 5 times
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Old 02-07-2010, 01:43 PM
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JD2439975 (Justin)
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Quote:
Originally Posted by luigi View Post
Justin,
Longer is always better.
When you can't expose longer you should use the highest ISO available.
I think I've already said this 5 times
Sounds good Luis, just wanted to hear it again.
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Old 02-07-2010, 02:05 PM
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Octane (Humayun)
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Justin,

I've always been a proponent for longer and lower.

With the newer cameras, the (perceived) higher ISOs can be used effectively as there is a lot less noise than the old days.

ISO-1600 ten minute exposures on my 5D Mark II yield such clean sub-exposures.

H
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Old 02-07-2010, 02:20 PM
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Ah H if only the 40D behaved so well.

Hate to think what 10 mins @ 1600 would look like on an old 350D, glow in the dark stuff I'll bet.
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