In the previous article about the best ISO setting for astrophotography, I’ve explained what ISO is, how it works in digital cameras and how to determine the best ISO to use for your DSLR.
However, theoretical discussions aside, the question is of course if we can even see the difference in reality? During my stay in Namibia I did a few ISO800 vs ISO400 tests, so I’ll share the results in this post. Please note that these are just examples to show the differences between the two ISO settings. At that time I didn’t have the level of understanding of the concept of ISO that I have now, or I would probably have done some different tests and for my Nikon DSLRs go all the way down to ISO100. However, if you are anything like me and it feels very counterintuitive to you to be using real low ISO settings for astrophotography, it will be quite interesting to see some examples that show the differences. Hopefully this will be another step that will help convince you to really look for the best ISO setting for your camera, even if this is a (very) low ISO setting.
Nikon D7000: ISO 800 vs ISO 400
When I was on my astrotrip to Namibia I did do a few shots on the globular cluster Omega Centauri. I took a few exposures at ISO 800 with 150 sec exposure time and a few shots at ISO 400 with 300sec exposure time. As you can tell from this, I was still ‘stuck’ in thinking in terms of the ‘exposure triangle’ and doubled the exposure time when I halved the ISO. But remember; the exposure triangle is a flawed concept and we really should also be considering the ISO as a setting on it’s own. Unfortunately I didn’t do that on this object, so let’s just look at the differences;
That is quite convincing. ISO400 is basically outperforming the ISO800 exposures in all areas.
But those are just numbers. Can we see the difference when we look at the individual frames side by side?
Please note that all the images below are stretched using the ScreenTransferFunction function in PI.
Hmmm, I can not really see a difference here. (Apart from the focus, which is not due to the ISO setting of course) But this is a difficult area of the picture to do a comparison on, let’s zoom in a bit further and look at a faint galaxy in the background;
The noise difference is hard do judge visually I think, but what becomes strikingly apparent is the brightness and definition of the faint stars. They are much brighter in the ISO400 picture. Intuitively this might still be strange, but considering the fact that the read noise for the D7000 is pretty much consistent right from ISO100 it completely makes sense. Basically you are capturing twice the amount of light, so sure the faint stuff becomes more apparent and brighter!
As said it would be better to look at the differences for exposures of the same length. So let’s look at a few exposure tests I did on the Helix nebula; ISO 800 with 600sec exposure, ISO 400 with 600sec exposure and ISO 800 with 300sec exposure.
First let’s look at the numbers:
Wow, look at the Noise, that’s a huge difference! And looking at the SNRWeight we can tell that we didn’t loose as much signal so we gained quite a bit of SNR by using ISO400 instead of ISO800, even if we compare it to the ISO800 with the same exposure time. The star support difference between 300sec and 600sec is obvious, the loss of 111 stars going from ISO800 to ISO400 might be statistical and not significant as it is only 2.23%.
Now let’s see if we can see the SNR difference in the images visually as well;
Hmmm, again the difference is not that obvious visually.
Maybe if we zoom in a bit more;
Well, the difference between the 300sec and 600sec can be clearly seen. However, the difference between the ISO800 and ISO400 in the single image is hard to tell.
It is clear that in order to get the faint signal, exposure is key!
Nikon D600: ISO 800 vs ISO 400
For my image of the Sagittarius Triplet I did a quick test for my 2nd and 3rd runs and upped the exposure time from 300sec to 600sec while dropping the ISO from 800 to 400. It would have been better to test both ISO’s at 600sec, but I didn’t know yet that at the time 😉
So let’s work with what we’ve got and see if we can spot the differences;
We see the same picture again; Noise is lower while SNR is improving as we lower ISO. This is clearly indicating that we are in the area where upstream read noise is swamping the downstream read noise, because otherwise we would have seen the reverse (SNR improving as ISO is higher).
So most likely we will have to go even lower for the optimal ISO for the D600.
Again, visually it is hard to see the difference. Only if we zoom in real close we start to see the difference. But remember, since we are in the ISO-less zone for the D600 at these ISO’s, we are really comparing exposure lengths and not so much the ISO settings.
Conclusion on ISO800 vs ISO400 comparison
For both the D7000 and the D600 it is clear that we are better of with using ISO400, even though it might not always be that clear visually. The numbers don’t lie, and we have to remember that we are comparing single subs here. The difference will become stronger when we start stacking, especially the dynamic range difference in terms of picking up the faintest signals.
Next time I’ll do a test on ISO100 and ISO200 versus ISO400, to see if I have to go all the way down to ISO100 on the D7000.