Star Trail Reflection

Sometimes when I present a star trail shot over water, some of the people who look at it think it's "fake" if they don't see a perfect reflection of the stars.  As you walk around in daylight, every time you see water does it reflectsomething?  No, not always.  When there is a reflection, is it as bright as your direct view of the same scene?  It may look that way to your perception, but no, photographers know that the reflection is much darker and contains much less light.  In fact, we often use graduated neutral density filters of 2 to 3 stops to balance out the direct and reflected images, so we know that the reflection doesn't just contain a little less light, the direct view of the landscape contains 4 to 8 times more light.  So how much light is in the reflection?  If it's 1/8th of the direct light, that's about 12%.

There are a lot of circumstances affecting how strong a reflected image is.  The angle of incidence makes a difference.  Light arriving at a small, low angle reflects better.  Think of a rock skipping across the surface: you throw a rock to skip down close to the water, and throw it almost parallel to the surface of the water.  Light bounces off the surface better at a shallow angle like that too.  So a low tripod height will help.  If you try shooting from a high tripod, the light at the foot of your tripod has to come almost straight up.  Can you skip a rock straight down?  No.  You can't reflect photons of light very well that way either.  Most of the light hitting the water at such a large angle to its surface simply passes into the water instead of reflecting.  In clear water, so much light is going in and out when viewed straight down near your feet, you'll typically see the bottom of the lake at your feet instead of a reflection.

But let's apply what we've already considered to this shot at night.  You see far more stars after the moon sets.  Many stars are barely bright enough to be seen when the moon is up.  My typical dark sky exposure is  30 seconds at f/2.8, ISO 6400.  The dimmer stars are visible in the sky, but when you cut their light 88% to leave the typical 12% intensity in the reflection, the camera's sensor is no longer sensitive enough at this exposure to pick them up.  You'd have to increase the sensitivity 8X to have a good chance of picking them up: ISO 51,200!  There are other factors such as the calmness of the water.  If that faint point of light moves even a little, it spreads out in the long exposure, and its brightness, its intensity, goes down.

For this image taken when no moon was in the sky, I captured 228 images of 30 seconds each, at f/2.8, ISO 12,800.  That compromise overexposed the sky and underexposed the reflection, so in post-processing I darkened the sky and brightened the reflection, but many stars were simply too dim to show up in the reflection at ISO 12,800.  So if you want to take a star trail reflection picture at night, break out your 3 stop graduated neutral density filter, set your ISO to 51,200 (if you want to keep the exposure times short), and I wish you luck! 

Edit: Here's my blog post on the subject of star trails:
Creating Star Trails Images
http://activesole.blogspot.com/2011/05/creating-star-trail-images.html 

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