10 mins at ISO200 vs 5 mins at ISO400

[BrendanC]

Which would be best? As per the title of this post?

I know different cameras have different characteristics, quantum efficiencies, resolutions, pixels sizes, well depths etc. And each object/location/night sky might require different settings. But in general, say I took 10 mins at ISO200, and 5 mins at ISO400 (even, for the sake of argument, 2.5 mins at ISO800)... which, generally, would be better? Which would be more likely to have a better S/N ratio for example?

I also know this is something I need to test for myself! And I will do (and have to an extent but not very deeply). But, given the scarcity of good weather I'm reluctant to experiment too much - I just want to take photos. So, if someone can definitively answer this, that would be great. I expect there's a mathematical argument for long exposures at low ISO versus equivalent shorter exposure at higher ISO?

Thanks, Brendan

Edited January 18, 2021 by BrendanC

[tooth_dr]

Brendan

Firstly apologies for always answering your threads!

Ive come across this question a few times and it does vary from camera to camera. Some cameras like the Nikon D5300 prefer 200, and others such as Canon 1000d this might be ISO 800.

In short answer to your general question - there is ‘noise’ in every image you take, including read noise.  Taken fewer images at a lower ISO might be better to maintain dynamic range but it’s a trade off with increased read noise.

Id stick with 800 for your canon and work your times around the sky where you shoot at  

 

[BrendanC]

Not a problem! Just so long as you're not stalking me...!

Totally take your point and this question has indeed been asked a bazillion times. The answer can only be as generic as 'long/high enough to beat the read noise and short/low enough not to clip stars'.

So, close eye on the histogram from now on then.

Thanks for the help! Again!

[wxsatuser]

On 18/01/2021 at 20:17, BrendanC said:

Which would be best? As per the title of this post?

I know different cameras have different characteristics, quantum efficiencies, resolutions, pixels sizes, well depths etc. And each object/location/night sky might require different settings. But in general, say I took 10 mins at ISO200, and 5 mins at ISO400 (even, for the sake of argument, 2.5 mins at ISO800)... which, generally, would be better? Which would be more likely to have a better S/N ratio for example?

I also know this is something I need to test for myself! And I will do (and have to an extent but not very deeply). But, given the scarcity of good weather I'm reluctant to experiment too much - I just want to take photos. So, if someone can definitively answer this, that would be great. I expect there's a mathematical argument for long exposures at low ISO versus equivalent shorter exposure at higher ISO?

Thanks, Brendan

Never double the ISO and half the exposure, they are not equal in AP.
Basically the total exposure should be as equal as possible.

One 10minute frame at ISO 200 is two 5minute frames at ISO 400 and four 2.5minutes at ISO 800, etc.

[MartinB]

6 hours ago, wxsatuser said:

Never double the ISO and half the exposure, they are not equal in AP.
Basically the total exposure should be as equal as possible.

One 10minute frame at ISO 200 is two 5minute frames at ISO 400 and four 2.5minutes at ISO 800, etc.

I agree with this.  You don't get any extra photons on the chip by increasing the iso.  Increasing the iso is simply increasing the amplification of the signal.  Some cameras vary in their response to different iso settings and you can read all sorts about this if you do a search for "iso invariance" and this can be an issue if taking single night time images, e.g. landscapes.  However if you are taking multiple sub frames you need to expose for long enough so that the basic fixed noise that arises from the camera reading the chip is obscured by sky glow.  Obviously this varies depending on sky brightness, but also read noise is lower at higher iso's.  So your minimum optimal exposure time will be longer with low iso.  However, once you are over the read noise threshold a lower iso gives you more room for manoeuvre during processing.  You can experiment using different exposure times at different isos but, if you can track for 5 minutes and don't have really dark skies this should see you easily see you over the read noise and you should be fine.