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 !!
hola jeff, muchas gracias por compartir tu experiencia!!!!! yo desde chile , ya he visto todos tus videos y estoy a la espera de nuevos videos o blogs, particularmente en este me queda una duda, haces solo una exposicion de 32 minutos?? no le pasa nada al sensor, o haces mas de una exposicion? yo lo maximo que me he atrevido a exponer es un tiempo de 15 minutos por temor que siempre se ha escuchado que se podria quemar el sensor, yo tengo una canon 6D
saludos y la mejor de las suertes
Hola Sebastian. Glad you like the videos. I do many lenghts of exposures. I prefer to do very long exposures instead of short ones and stack them together because they look more real and are better because of the lower ISO that can be shot at. I do not have any problems of getting “hot pixels” or of burning up the sensor. At least not yet and I hope I won’t. Thanks for asking
grecias a ti y tu humildad te hace mas grande….. exito en todo y la mejor de las suertes, comentame si existe la posibilidad de enviar a chile un libro tuyo . saludos
Sebastian; Gracias. Yes, any of my books can be sent to Chile. Order them online from the store.