This image was taken in January of 2016 while out shooting in the “Glacier View Area” a 2 1/2 hour drive northeast of Anchorage. I visualized making an image like this before making the trip. The idea was to make an image of a winter mountain and snow landscape with head and/or tail lights on the highway. During daylight hours I scouted for this scene (and other night-sky vantage points). It’s always best to scout locations ahead of time, during daylight in order to actually see the foreground, find a safe parking place and confirm there are no obstacles that won’t work … like power lines that are very difficult to see at night. I just did not know where I would make it or how it would turn out. I”m very happy with the results. Here’s the method I used to make this image work.
First, I had to determine just how long the exposure needed to be in order to have the car lights streak the entire length of the highway. To do that, I waited until headlamps appeared in the far distance, just cresting the hill. I hit the stopwatch on my smart-phone. It took that vehicle 2 minutes and 15 seconds to make the trip. So, I determined that I would make an exposure of 3 minutes in order to be on the safe side, in case another vehicle took a longer time.
The first image I made was just to determine the exposure for the ambient light, not the car lights. There was no car passing at the time. I used “Aperture priority” metering at a “+ 1 stop”, f4, ISO 400, which yielded an exposure of 15 seconds. A careful check of the histogram showed that this was, indeed, a good exposure. The highlights were not clipped on the right and the shadows were not blocked up on the left. Like this:
It does not always happen that way and if the histogram was not correct, I would have adjusted the “exposure compensation” to the PLUS or MINUS side, depending on which way the histogram needed to move.
Knowing that this exposure was correct, I then manually calculated, on my fingers, what I would need to get a 3 minute exposure. I did this first by taking the current 15 second exposure and doubling it until I reached 3 minutes. Which in this case was as follows:
15 seconds doubled to 30 seconds is ONE stop more of light than 15 seconds.
30 seconds, doubled to 60 seconds (1 minute) is TWO stops more of light than 15 seconds.
One minute, doubled to 2 minutes is three stops more of light than 15 seconds.
2 minutes, doubled to 4 minutes is 4 stops of light… I only need 3 minutes, so a 3 minute exposure would make it 3 1/2 stops of light.
Knowing that I was going to use 3 minutes of exposure I then needed to adjust either the current aperture of f 8 OR the current ISO of 400 by 3 1/2 stops of LESS light. I then first changed the ISO setting because, ideally, I want to shoot at the lowest ISO I can get in order to have an image with the least amount of noise. So, the math on changing the ISO went like this:
400 ISO, divided in half is ISO 200 which is now one stop of light less than ISO 400
ISO 200 divided in half is ISO 100 and now two stops less light than ISO 400
ISO 100 divided in half is ISO 50 and now three stops less light than ISO 400
ISO 50 is the LOWEST ISO my camera has. But I only have 3 stops of light less than I need. I need to get another 1/2 stop less light. So I took the aperture from f 4.0 to F 5.0. To get 3–2/3 stop less light. My final exposure for this scene was 3 minutes, ISO 50 at f 5.0 on a Canon 5D mark III with a 24-105mm f4 lens at 28mm on a tripod. And here is the histogram for that:
I made many exposures of this scene in order to get variations on the light streaks as I changed the focal length of the lens in order to get some variety. All the while, the light was getting less and less. As the light got less, I increased the exposure time to compensate and reviewed the histogram. Some vehicles had too bright of lights, some vehicles were only going away from me, some were big & tall trucks with high lights. The image above is my favorite, as there just happened to be a vehicle with bright lights that lit up the trees and some snow and frosted willows in the foreground very well. A little serendipity goes a long way in photography. Below are few that were good, but I did not like as much as the above.