It is common practice among photographers to set the ISO to AUTO. Although this is useful in many scenarios (Example: wildlife, landscape photography, etc.) giving the ISO “free will”, in this instance, can prevent you from capturing the beautiful colors otherwise only visible with your eyes. That been said, keep in mind that there is no right nor wrong way of capturing sunsets. At the end it is all about personal taste.
When photographing sunsets I like to set the ISO to 50 (or 100). By setting a low ISO I am making sure that the camera won’t compensate the lack of light for me.
I tend to use a slow shutter speed somewhere between 60 – 125. Using a higher shutter speed will result in even less light considering that I am using the lowest ISO possible. At the same time, using a slower shutter speed will allow more light but it almost requires a tripod (or very solid surface) due to the blur produced by the longer exposure.
Aperture is key. I use it to set the exposure that, at the end, will give me the desired result. The secret is to move the aperture to one or two steps below the proper exposure so that I can capture all the fading colors. Lowering the exposure will also result in silhouettes which I absolutely love!
When purposely lowering the exposure my goal is no longer to capture detail but color and silhouettes.
As mentioned at the beginning of this article there is no right nor wrong way of capturing sunsets. Playing around with the different elements involved in capturing light (ISO, Shutter, Aperture, Exposure) will break nothing but the frontiers of your imagination.