Today’s Question: When shooting at night, is it better to shoot for a “proper” exposure at a high ISO, say 3200, or underexpose a couple stops with a lower ISO, then compensate in processing with increased exposure?
Tim’s Quick Answer: You will get the best results by achieving the brightest exposure possible without clipping highlight detail while at the same time using the minimum possible ISO setting. If you need to achieve a shorter exposure duration, generally speaking you are better off increasing the ISO setting rather than creating an underexposure.
More Detail: When you raise the ISO setting, in a way you can think of your result as being underexposed based on, for example, a shorter exposure duration. The resulting image is then brightened up through the use of amplification of the signals recorded by the image sensor.
With the various cameras I have had the opportunity to test, the results consistently show that it is better to let the camera brighten the image through a higher ISO setting than to apply brightening to the image after the capture. In other words, the in-camera amplification of the signal recorded by the image sensor yields higher quality than applying the same change in brightness with software after the capture.
So, if at all possible I would use the lowest ISO setting available to minimize noise, and create an exposure that is as bright as possible without clipping highlight detail (or only clipping the brightest areas, such as illuminated lights). If I needed a faster shutter speed (shorter exposure duration) for any reason, I would raise the ISO setting in order to achieve that goal, because this will generally provide the best final image quality compared to underexposing the scene and then brightening the image later.