Today’s Question: I’ve read some discussions on the night sky photography sites, in talk of noise, about the thought that you can potentially get lower noise by shooting the night sky at a lower ISO, maybe 1250 or 1600, and then adjusting exposure in post processing, as opposed to just shooting the image at a high ISO, say 6400. What are your thoughts?
Tim’s Quick Answer: In general I recommend increasing the ISO as needed, but not under-exposing the image. The optimal settings in terms of noise will vary from one camera to another, but in general you will get better results raising the ISO setting than under-exposing and brightening in post-processing.
More Detail: There are some special exposure requirements when it comes to night sky photography, such as ensuring your exposure time isn’t too long in order to avoid the appearance of star trails in a photo. But regardless, noise becomes a very real concern.
In general you want to use the lowest ISO setting possible, but not at the expense of a good exposure. I would typically start with the lens aperture wide open (or nearly so), determine the longest shutter speed possible to avoid star trails, and then adjust the ISO setting as needed to achieve a good exposure that is as bright as possible without blowing out highlight detail.
I also recommend enabling in-camera long exposure noise reduction, as I have generally found this to be extremely helpful with night sky photos. When possible, try to allow some time in between exposures, to avoid letting the camera get too hot. When the camera gets hot, noise levels can be significantly increased.
Generally speaking you will find that raising the ISO setting in the camera will result in less noise than using a lower ISO to create an under-exposed photo that must be brightened in post-processing. But again, different cameras will behave differently, so it is a good idea to test your specific camera at a wide variety of settings so you can get a better sense of the noise performance of the camera you’ll be using for night sky photography.