In a previous article I explained in detail what ISO does in the camera and what, in general, is the best ISO value to use for astrophotography.
It all comes down to finding the ISO value that is the best mix of read noise and dynamic range. Some might wonder about unity gain as well, but you can read here why you shouldn’t worry about unity gain.
Basically it comes down to finding the ISO value from which the upstream read noise will swamp the downstream read noise. Any ISO value beyond this one has no additional benefit and only decreases Dynamic Range. Remember; ISO does not increase the sensitivity of the camera, increasing it doesn’t capture more light while decreasing dynamic range will hurt our images for instance in star color.
I stated that you can go to sensorgen.info and look at the read noise chart for your camera. These charts are not always really clear and there is a better way to show which ISO value you want to use; graph the dynamic range versus the ISO values (on logarithmic scale).
You can check the graph and look for the point in the curve where the linear trend starts. This indicates the point at which the upstream noise swamps the downstream noise and all we do is amplify the signal and the noise, hurting our dynamic range. These are the noise values where we call a sensor ‘ISO-less’. A lot of Nikon cameras are basically ISOless and therefor will show a linear graph right from the first ISO values.
Suggested ISO values
I want to emphasise that the listed ISO values is the suggested value based on the available Sensorgen data. You should always check this data for your specific model and test it to see if it is right. For some models it might be less clear what the right value is than for others. Furthermore it is possible that there are differences per camera of the same model. You should take this value as a starting point for your own test.
Furthermore it is important to note that there is no benefit in increasing ISO higher than the suggested ISOs, however in some cases you might want to use a lower ISO value for even more Dynamic Range. This could be the case for instance when you expose long enough and the read noise is swamped by the background shot noise and you get the histogram to 1/3 from the right.
Please let me know in the comments below if you think a suggested ISO value is incorrect!