How to set up a long exposure:
This is a set-up shot for the long exposure star trails, and is a great example of why you would want to know how to calculate equivalent exposures in your head. When you are making a commitment to an exposure of any significant duration, how do you know what settings to use without wasting a lot of time? Well, if you know how to calculate equivalent exposures, it’s easy, and takes no more than 30 seconds (or 1 minute if you are using Dark Frame noise reduction).
In this shot, my settings were ISO 6400, f/2.8, 30 seconds. I found with these settings, the sky was nicely exposed, and our campfire was providing just enough light for the waterfall and trees in the foreground. Since I knew I wanted to shoot at ISO 800 or less, my equivalent calculations started there. I started counting stops from ISO 6400 to 3200 (1), 3200 to 1600 (2), 1600 to 800 (3). Next, I knew for the best star trails I wanted to make the exposure as long as possible, so I chose to adjust my shutter speed to balance my exposure. I went from 30 seconds to 1 minute (1), 1 minute to 2 minutes (2), and from 2 minutes to 4 minutes (3). At this point, my aperture setting of f/2.8 was not going to give me the depth of field I was looking for ( A daytime landscape might call for an aperture of f/8 or higher for maximum depth of field, but on a very dark night such as this, f/5.6 is usually sufficient). I dialed in f/5.6 and adjusted my shutter speed 8 minutes. For good measure, I added one more stop to my shutter speed making it 16 minutes to make sure I was filling in the shadows in the foreground. (1 stop over exposure is well within the corrective capabilities of Lightroom, so by exposing to the right, I was making sure I wasn’t going to end up with blocked up shadows). As it turned out, I let it cook for and additional 2 minutes beyond my calculated time while I finished my cup of hot tea. 2 minutes makes very little difference in the overall exposure as the next full stop of light would come at 32 minutes.
So, in a minute or less, I knew what my settings should be for an exposure of 16 minutes. I could have easily made it 32 minutes by changing my aperture to f/8, 64 minutes at f/11, or 128 minutes at f/16. All would have had the same basic exposure, but by setting up my initial test shot, I didn’t have to waste any time to find the right combination.