Which of these M27 individual subexposures is likely to stack better?

[R1k]

I am shooting with a Canon 800d and SW ED80. apparently the sweet spot for my DSLR is ISO200. Below are 2 single, cropped, 5 minute guided subs at ISO200 and ISO800 using a mounted L-eNhance filter.

Which is likely to stack better? The ISO800 image clearly has more signal so I feel reluctant to use ISO200 despite it being the optimum ISO for my camera

 

thanks in advance 

[rnobleeddy]

Assuming you didn't scale either image, then I'd guess that the ISO 200 shot, because that has been proven to produce the best results. The bottom image looks better because its brighter but presumably as a lower signal to noise ratio.

Or to know for sure, run a test!

[ollypenrice]

It's always best to do your own testing. Sometimes I find this confirms the received wisdom but sometimes I find it doesn't.

Olly

[gorann]

To be able to campare them you should stretch them (e.g. with Levels or Curves in Photoshop) to the same brightness and look at the noise level. The one showing the least noise when you zoom in will be the best. If you upload a raw exposure here of each ISO I assume someone could have a look for you if you want a second oppinion. Maybe someone like Vlaiv @vlaivcould even give you a number for the signal to noise ratio.

[vlaiv]

12 hours ago, R1k said:

Which is likely to stack better? The ISO800 image clearly has more signal so I feel reluctant to use ISO200 despite it being the optimum ISO for my camera

Just because something is rendered brighter on the screen does not mean it has more signal!

ISO setting is just multiplication factor.

You used same scope and same exposure time and same camera for both of these two images. Images received same number of photons - same signal.

If you have say 100 photons and you use multiplication factor of 8 in one instance and get ADU value of 800 and multiplication factor of 2 in second case and get ADU value of 200, does that mean that somehow signal got stronger?

That is simply brilliant - all one needs is single exposure and the we just multiply with very large number and get very strong signal! No need to spend hour sand hours under the sky!

However, that is not the case. It is not numerical value that we assign to signal that is important - it is signal to noise ratio and that one is fixed to number of photons we captured. SNR is equal for both images (at least that part coming from photons).

Why the recommendation to use ISO200 then? It has to do with other noise sources, particularly read noise. ISO200 is probably "sweet spot" - as ISO gets larger read noise gets smaller (good thing) and so does full well capacity (not good thing). ISO200 is likely to be the best balance of the two - giving lowest read noise for highest full well capacity.

In the end - when you stack your images, it is highly unlikely that you'll be able to tell the difference between the two - ISO200 and ISO800 as resulting noise difference is so small and can only be measured and not perceived by eye. Shoot which ever way is more convenient to you.

[R1k]

1 hour ago, vlaiv said:

Just because something is rendered brighter on the screen does not mean it has more signal!

ISO setting is just multiplication factor.

You used same scope and same exposure time and same camera for both of these two images. Images received same number of photons - same signal.

If you have say 100 photons and you use multiplication factor of 8 in one instance and get ADU value of 800 and multiplication factor of 2 in second case and get ADU value of 200, does that mean that somehow signal got stronger?

That is simply brilliant - all one needs is single exposure and the we just multiply with very large number and get very strong signal! No need to spend hour sand hours under the sky!

However, that is not the case. It is not numerical value that we assign to signal that is important - it is signal to noise ratio and that one is fixed to number of photons we captured. SNR is equal for both images (at least that part coming from photons).

Why the recommendation to use ISO200 then? It has to do with other noise sources, particularly read noise. ISO200 is probably "sweet spot" - as ISO gets larger read noise gets smaller (good thing) and so does full well capacity (not good thing). ISO200 is likely to be the best balance of the two - giving lowest read noise for highest full well capacity.

In the end - when you stack your images, it is highly unlikely that you'll be able to tell the difference between the two - ISO200 and ISO800 as resulting noise difference is so small and can only be measured and not perceived by eye. Shoot which ever way is more convenient to you.

Thanks so much for your very detailed and excellent explanation. Appreciate the help