You know sometimes your mind gets blown with an “of course!” moment, which by its very nature leads you to discover something you realize you could have figured out on your own? I’m always a little embarrassed by those, since – well – I should have known. But they can still be cool enough to share, despite the embarrassment..Maybe I’m the last to discover this, but, polarized filters work at night.“What?” Some people are saying. “Of course!” You might be saying.For the benefit of my own narrative, I’m going to explain why. For the last few nights I have been on a favourite river of mine. The moon is full right now, so the light was bright, shadows everywhere. I decided to to some night exposures, since I haven’t played with the bulb mode on my A55 very much yet. I wanted to get some star trails, among other things, but I discovered a few things.The first night I aimed my camera at the sky, despite the light pollution from the moon. Previous exposures I’d done with stars totalled about three minutes. Enough to get a short star trail. I aimed high and kept a few pine trees in the frame, for composition – slightly worried about the moon overexposing them. That’s not what happened. But in this six minute exposure I saw a few things I’d seen before to lesser degrees. Colours came back. Of course colours are still there at night, but it’s always creepy to see them reemerge in pictures taken in the dark, with perfect reproduction to boot. The other thing was the sky. It was still blue! I think we inherently know this. After all, the phrase midnight blue refers to something right? What I didn’t realise is that the color balance of moonlight is not so different from that of sunlight – it’s only dimmer! After this long of an exposure, everything reemerged. This was the first piece of the puzzle. The blueness of the sky is controlled by the polarized properties of sunlight. This means that if moonlight makes it blue too, then moonlight must also be polarized. Again – of course! The moon reflects the sun anyway, right?
In the presence of moonlight, the blueness of the sky is preserved.
The next night I was further downriver, thinking “I wonder what a seven minute exposure of a flowing river would look like.” I walked down to the river, and the moon was right in front of me, polluting out the starlight, as before. Instinctively, I screwed on my polarizer and gave it a turn. Through the viewfinder I could see the glare getting cut. Unreal. There is more experimentation to be done here! This means that the brightness of the stars vs the sky can be modulated with a polarizer too! I can’t see them through my viewfinder, it’s digital. Maybe you can see them through your optical one, but both of us are going to have to experiment more to see what the result on the blueness is, but it should be comparable to sunlight. Meanwhile, I got a shot into the moon with reduced glare, and got my original question answered about long exposures and rivers at night. The water didn’t look liquid any more – I’m really excited about the potential of this effect, as I haven’t seen it widely used (in exposures longer than a few seconds of course).
This is the shot with the polarizer working. Without it the glare covered most of the river.
After some cursory googling, I also couldn’t find much about polarised filters at night, except a lot of people saying it was a dumb idea to lose that many f-stops in the dark for no reason.
PHYSICS! I say.