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How do you choose ISO?


Swillis

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Hi, pretty new to AP and I was interested in how everyone chooses what ISO to use?  Is this more camera/scope/lens or target based? I'm guessing LP will have a big part to play here also?

I have a Canon 350d with a (kit) 75-300mm lens on a star adventurer. ISO 800 (maximum is 1600).  Using what I have to get started...

From what I understand ISO shouldn't matter for signal(?) As it effectively multiplies the signal, same number of photons reaching the detector. But also read noise goes down at higher ISO?  So we should use highest possible ISO

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13 minutes ago, Swillis said:

From what I understand ISO shouldn't matter for signal(?) As it effectively multiplies the signal, same number of photons reaching the detector. But also read noise goes down at higher ISO?  So we should use highest possible ISO

Thats pretty much it, but with a few caveats. Read noise goes down as ISO increases, but for most models only to a point. Once at that point there is little to gain from pumping up the ISO, but there is detail to lose in the form of overexposed pixels. Fully saturated pixels no longer contain useful information so this should be avoided, if possible.

Where this sweet spot lies depends on the model of camera in question, but according to some sources the 350D sweet spot would be at around ISO400. For my 550D it is around 800 and that's what i used for all shots when using the camera. http://dslr-astrophotography.com/iso-values-canon-cameras/    https://astrophotography.app/EOS.php

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10 minutes ago, ONIKKINEN said:

Thats pretty much it, but with a few caveats. Read noise goes down as ISO increases, but for most models only to a point. Once at that point there is little to gain from pumping up the ISO, but there is detail to lose in the form of overexposed pixels. Fully saturated pixels no longer contain useful information so this should be avoided, if possible.

Where this sweet spot lies depends on the model of camera in question, but according to some sources the 350D sweet spot would be at around ISO400. For my 550D it is around 800 and that's what i used for all shots when using the camera. http://dslr-astrophotography.com/iso-values-canon-cameras/    https://astrophotography.app/EOS.php

Thanks for the swift reply they look like very useful resources. 

Looks like 400 it is then. 

👍

 

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7 hours ago, Elp said:

Generally the lower the better to reduce unwanted noise, higher to get more "sensitivity". For starfields I've found the sweet spot iso 800-1600. You'd have to try a few and see the differences.

Thanks Elp,

What camera is that with?

I guess with most things in this hobby a bit of experimentation is required, but enough clear nights would be helpful to try things out😅

A good starting point is always helpful 

Thanks 👍

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@Swillis I have a Canon 600D which has the IR filter removed so it captures more IR bandpass light, the rotating screen I find is a godsend for AP as a lot of times the scope is pointing near vertical and I have my setup close to the ground with a short tripod. @ONIKKINEN thanks for explaining it better, why I used " " the higher Isos look like they're boosting sensitivity but the overall image just looks brighter with more noise though it depends on the camera model where this value peaks. Iso used to be more critical back when you had to choose the grade of film to use, with digital post processing the process can be more forgiving.

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Owning but not using a 350D for a long time and never for astro, all I can say is shy away from high ISOs. Being an older model they're not exactly stellar performers when you ramp it up.

The general rule with cameras of that era was to keep the ISO as low as possible to minimise noise and image degradation. Even 400 might be pushing it too far.

I've accidentally left the ISO high and even daylight and sunny day pictures have been horribly grainy looking with my 350D 

Edited by reezeh
"for a long time" in post
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@reezehYes certainly is an older model. Had it for a long time, and know that high ISOs were very noisy even as you point out in daylight. Maybe I'll get to upgrade one day, but for now it's serving me well as I get started in AP. 

I think the links @ONIKKINEN provided give me a good rational starting point (as a scientist I like a nice graph!)

Thanks

 

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