Monday, March 30, 2009

The Case of Aurora and the Mysterious Rings

I was stumped with a good photography question over the weekend. I was asked why a series of tiny rings developed in the center of a digital photo while shooting long exposures of the northern lights (aka Aurora Borealis). I had seen similar distortions years ago, while scanning some large-format negatives with a flatbed scanner - but that didn't give me a lot to go on, except that it was the result of some reflecting light or concave distortion.

Here's one of the images I was shown. It's hard to see the distortion as this is compressed like crazy for the web, but I assure you... there's some rings center frame:


Here's a close-up of the distortion:


After scouring the web for any clue as to what could possibly cause a problem like this, I came across a forum on DPReview. It seems as though the problem is caused by light reflecting off of an inner lens elements back towards the UV filter. The rings are actually the reflecting light off of the UV filter's inner side. The distortion itself is generally known as Newton's Rings.

The most simple solution is to remove your UV filter while shooting the Aurora lights, although a really high-end filter (B&W, Leica) may also eliminate the problem. Some even recommend removing the UV filter when shooting at night, no matter what the subject matter is.

For you math nerds out there, here's the formula for why it happens:


Yep, that pretty much explains it.

So in short: if you're shooting Aurora, remove your UV filter. A little knowledge that just might come in handy when you least expect it.
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