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.
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.
We can look at Sensorgen data to determine this point, although this is not easy to see from the charts in all cases.
So I decided to create graphs which are able to show this ISO value really clear. 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’ and there is no benefit in increasing the noise in this ISOless range.
It is good to note and realise that, in general, Canon cameras require a higher ISO value to reach this ISOless range than Nikon cameras for instance. So if you ever find yourself in a debate about which ISO you need to use, it is always good to know which brand and model you are discussing.
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.
Canon cameras can suffer from banding noise and you might find that you need a higher ISO even than listed below to cancel this out properly.
Furthermore it is important to note that there is good reason 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.
You shouldn’t worry about unity gain. For all suggested ISOs listed below, the read noise is (well) above 1 LSB so the ADC will be dithered properly. You can find more info here on unity gain and why you shouldn’t worry about it.
Please let me know in the comments below if you think a suggested ISO value is incorrect or if you are missing a camera model in the list!