Night Shooting Math and other Tips

I want to thank the sizeable group who came out last night for our Night Photography workshop.  Sadly the clouds conspired against star trails, star points and the Milky Way.  Attendees did however experiment with light trails from the cars on the highway, some light painting and I saw really interesting images made by Heidi and Bryan who stayed past 11 when the super-moon started to move from behind the clouds.  The sky textures were really interesting.

I also saw some very interesting images combining the light trails with the dusk sky and commend everyone for using the workshop as it was meant to be, a time of trying new things.

At the beginning, I shared a couple of math tools that anyone can use to help build a night shot, without spending a long time and perhaps hours in testing mode.

The Rule of 60

There are 60 seconds in one minute and this allows for some interesting math.  With only the Rule of 60, your shutter speed and your ISO settings, you can see quite quickly what a really long exposure will do. 

If you want to see what a 1 minute exposure at ISO 100 will look like, do the following.  Divide the time in seconds by sixty and multiple the ISO by 60.  This will give you your test model.  So in this example, 60 seconds divided by 60 is 1 second, and 100 multiplied by 60 is 6000.  Round to the closest usable exposure, in this case 1 second at ISO 6400.  This way you can see what your desired exposure will look like without having to do a long exposure test.  This really comes into its own when you consider what the outcome of a 15 minute exposure at ISO 100 would be.  You want a low ISO for low noise and best colour fidelity, but don't want to make a 15 minute test.  Use The Rule of 60 and make a 15 second exposure at ISO 6400.  This exposure will show you what 15 minutes at ISO 100 will look like.

The Threshold to Keep Stars as Points

We all know that the earth is turning.  We know that stars will occupy different positions in the sky as the night progresses.  But how do we know how long we can make an exposure and still keep the stars as points of light?  For this we use the magic of 600.  To find out the maximum exposure duration for any focal length to keep stars as points, simply divide 600 by your focal length.  For example, if you are shooting at 18mm focal length, divide 600 by 18.  You get 33.33.  Round that down a bit and you know that at 18mm you can make a 30 second exposure and the stars won't appear to move.  If you were shooting with a 200mm focal length, the math tells you that the stars will start to streak after 3 seconds.  Very simple.

Some Other Night Shooting Tips to Remember

If you don't know where to start, eliminate the aperture as one of the criteria.  Use the old National Geographic guideline, f:/8.0 and be there.  Certainly vary your aperture as you like but this is a great place to start.  Workshop attendees discovered very quickly that they got decent dusk skies, and nice light trails with 30 second exposures at ISO 100.

Shoot in RAW.  This eliminates significant loss of data in JPEG conversion and will keep noise down that results in the compression process.

Set your camera white balance to Auto.  There are so many potential sources, that no one setting will be right.  By shooting in RAW, no white balance is locked in, and be setting AUTO, the JPEG preview on the LCD won't be coloured oddly.

Set the Picture Style to Neutral.  Picture Styles only impact JPEGs as well as the JPEG preview on the LCD.  By choosing Standard, or Neutral, you don't apply any artificial changes to your preview.

Wide angle lenses have more depth of field at any given aperture than telephoto lenses.  Because you will be in manual focus mode, and because it's sometimes hard to see perfect focus in the dark, use of a wide angle lens improves your odds of success.

If you want the kind of round star trails, you already know that you want longer exposures.  It is common practice to want the trails to rotate around something and for those of us in the Northern hemisphere, the popular and simple answer is to include Polaris, the North Star in the frame.  You can find Polaris in the Little Dipper constellation, or draw an imaginary line from the two front stars in the bowl of the Big Dipper, or you can use a smartphone or computer application to map the sky.  I use an app called Star Walk that uses the built in GPS and Compass in the iPhone to overlay the night sky on the screen making it easy to find certain constellations, stars, planets and other celestial objects.

With star trails, rather than a single 30 minute exposure, you may choose to make sixty 30 second exposures.  This requires Photoshop to do the stacking of the images after the fact and is made easier with an intervalometer to trip the shutter.  Stacked images tend to have less noise and be less susceptible to backscatter from light pollution.

With very current cameras, Long Exposure Noise Reduction is often not needed until your exposures hit 10 minutes or more.  Canon Explorer of Light Jennifer Wu explains this in her night photography class.  I tend to leave mine on normal which on the 1Dx doesn't arbitrarily make the black exposure as long as the original, so it is less time consuming.  Experiment with your own camera to discover what works best for you.

Carrying a flashlight with a red gel helps you to see camera settings as well as where you are stepping without destroying your night vision.  As it takes over ten minutes for your night vision to fully develop this is very important.

Thanks to all who came out for the workshop.  This is the season to shoot the night sky, particularly if you can get out where there is little light pollution because on a clear night, the Milky Way will really standout.  Don't be discouraged by weather.  We had much more cloud cover than was forecast last night, but by 11pm the moon had risen high enough that it was regularly peaking through high cirrus clouds allowing for very atmospheric shots.

Ross Chevalier

Technologist, photographer, videographer, general pest

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