iso vs exposure vs....

[mislav]

Hi, what do you prefere? How hight to go with iso? I have sony nex5n, but my telelens is slow (min F5.6). I know that exposure is better, but i cant do good trailing fith this setup (heavy lens on nanotracker). I can do only 30sec good exposures. So what do you think where is limit to go high with iso? And is it better to kill lots of time and shot lets say 100 30sec picuters with iso 1600, or 30 30sec pictuers with 12800 iso... Will noise is going to be so bad to destroy image or???

[vlaiv]

ISO will not contribute to SNR it can actually hurt it if set improperly.

ISO setting is like multiplier - so if you have signal, let's say five times as large as the noise - and you multiply values in your image with two - both signal will be multiplied by two and noise will be multiplied by two - ratio of signal to noise will still be 5.

For good clean image you want ratio of signal to noise to be as large as possible. Two ways to get your SNR to be higher - longer exposures or more subs to stack. It all comes down to total integration time - longer total integration time for specific setup - better SNR. There is however difference between fewer longer subs vs more shorter subs (both totaling same integration time). Fewer longer subs is always better than many shorter subs (again for same total integration time). It is not easy to calculate how much better SNR will be with longer exposures (it depends on read noise, light pollution, ...). If you have limit on exposure length due to your equipment (to avoid stars trailing) - go with longest exposure that give you good subs, and just shoot as much of such exposures that you can (for some targets to get really good SNR you will need to do multiple nights of imaging).

On the matter of ISO - it depends on camera used, but for most DSLRs, ISO values of 800 or 1600 are best choices - you can search the internet for best ISO for your particular model.

[prusling]

There's a great video by Craig Stark on YouTube covering all the ways you can improve SNR. Search for "what do all great shots have in common"