The making of star trails

The Back Story to Making This Image—

This past spring I saw that the Aurora forecast was supposed to be good for our area… and the sky was going to be clear with no moon. So my wife Joan and I headed out to the Matanuska Valley to a location I like and know fairly well. I was happy to see the water was completely thawed from the winter. It was oh-so quiet and oh-so very dark without the moon out. The stars reflecting on the calm water was ethereal. And the amount of stars we could see seemed like a LOT more than we ever had. It was a magical time and we were enjoying our time in the outdoors.

One thing I typically plan to do when I go Aurora shooting is to shoot star photos while I wait for the Aurora. This serves me two purposes. One, I have a chance to get my location, compositions and to a certain degree, my exposures pretty much dialed in.  And second is that I get to make some other interesting photos while I wait…stars and star trails are fun to do. Variety for me is good. When shooting night-sky photography I almost always have two tripods and cameras set up, and many times I’ll have three setups going.  Perhaps one of them being a time-lapse. With several setups (typically various focal length lenses)… like a 14mm f/1.8 on one, a 16-35mm f/2.8 on another and a 24mm f/1.4 on the third, I can work the scene quickly and efficiently depending on what materializes as I shoot. It can also get quite crazy remembering what’s going on with which camera…but that’s part of the fun I think. Keeps my brain in high-gear.

So this was no different. I had two setup going. One was making real long exposures like the one below and I was using the other to do short exposures… mostly of the stars reflecting in the water.

This image was shot on a Canon 5DMark III with a 16-35mm f 2.8 lens at 16mm f2.8 ISO 100 for 32-minutes.

How does one calculate the exposure for a 32-minute photo like this— or longer for that matter?  Here’s how: Take a “high ISO test shot” in order to find proper exposure at a short exposure time and then extrapolate from there.  Here’s the method I use for this which works every time.

1.  — For my high ISO test shot, I set the camera to the “M” manual setting and use a 32-second exposure time at an aperture of f/5.6 and ISO 3200.   I shoot a frame and then I look at the histogram.  If it’s underexposed (too far to the left) I will open up the aperture to f/4 and do another test.  If still under exposed, I continue to open up the aperture (or increases the ISO if I can’t open the lens aperture wide enough) and make another test exposure until I have the correct exposure according to the histogram.  If it’s overexposed, (too far to the right), I will adjust the ISO downward… to 1600 and do another test until I have the correct exposure. 

NOTE:  Because the night sky and foreground is often much darker than 18% medium grey, a good histogram for most night-sky photography typically DOES NOT look like a properly exposed daylight exposure. A night-sky histogram is likely to look like something like this— depending of course on just how much dark/black vs. lighter subject matter is in the frame.

2.  Once I have my high-ISO test shot confirmed it’s a good exposure then it’s a simple matter of doing the “math” to calculate a longer exposure.  In order to calculate the “math” of exposure it’s necessary to know understand what makes up a “stop” (this is not f/stops— it’s stops ) of light for ISO, Aperture and Shutter Speeds. A stop is the doubling or halving of the amount of light let in respect to exposure when taking a photo. 

Below, each of these numbers represent a “stop” of light … either less or more light:

<<<<——LESS LIGHT  MORE LIGHT ——>>>>>

ISO —        50 – 100 – 200 – 400 – 800 -1600 – 3200 – 6400 – and so on and on.

Shutter speed —  ….1/1000 – 1/500 – 1/250 – 1/125 – 1/60 – 1/30 – 1/15 – and so on and on.

Aperture – F/Stops  … 16 – 11 – 8 – 5.6 – 4 – 2.8 – 2 – 1.4  and so on.

If you’d like to study stops of light in a more in-depth way, read this good article by John Rowell — http://www.john-rowell.com/blog/2017/3/27/what-is-a-stop-of-light

3.  So, for this case above, my high-ISO test was a proper exposure at 30 seconds at f/2.8 at ISO 6400.  So now I do the math, knowing that I want a 32-minute exposure.  And I honestly use my fingers to do the counting when I’m out in the field. I’m thankful I don’t have to use my toes too 🙂

The math looks like this… (doubling the time for the exposure is 1-stop of light).

* 30 seconds  to 1 minute (1-stop) to 2 minutes (2-stops), to 4 minutes (3-stops), to 8 minutes (4-stops), to 16 minutes (5-stops), to 32 minutes (6-stops).  I will increase the exposure time by 6 stops (from 30 seconds to 32 minutes)…

* and if I INCREASE the exposure by 6 stops I must DECREASE the exposure (less light) by the same 6 stops from somewhere else (ISO or Aperture) in order to balance out the proper exposure.  And I do that by using the ISO — because a lower ISO is always better to use to decrease noise.  So that math is:  ISO:  6400 to 3200 (1-stop), 3200 to 1600 (2-stops), 1600- to 800 (3-stops), 800 to 400 (4-stops), 400 to 200 (5-stops) and finally 200 to 100 is 6-stops of light.  BINGO !!