Outdoor Photography Canada

A very useful article by Dr. Robert Berdan. One thing not mentioned that might be of critical importance, however, is NOT to use any type of filter when taking exposures of the northern lights.

I shot some northern lights two nights ago, and was alarmed to find every image had a series of perfect, concentric rings dead center of every shot. I thought that it must be a sensor malfunction, because the rings were so perfect as to appear to be man made artifacts. They appeared in every image.

A Google search revealed that use of any filter, even a clear skylight filter (which I was using) results in a very unusual phenomenon wherein the specific light from the spectral green of the northern lights creates a light refraction between the lens surface and the fliter, creating a light beam bounce as though the two surfaces were mirrors in parallel, creating the image of concentric rings through the middle of your hear-earned timed exposures.

I was lucky, my shots were taken about an hour from home, but if you laid on big money for a northern trip and came home to find these rings in all your shots (on the rear display screen of my Canon 7D it seemed like the plastic of the display screen was going a wee bit of an odd reflection; in 6 years of shooting digital, I've never seen anything like it.

Borrowing from the outstanding Alaska Photograhy Blog: 

"TAKE OFF YOUR LENS FILTER

When photographing the aurora it is important to remove the filter from your lens. Why? Look at the photo at right and you will see a series of concentric rings, which appear at the center of the image. This can be a disheartening discovery after a night of shooting the aurora, since the rings are very difficult to remove, with even the best photoshop geek on the job.

What causes the rings? Charles Deehr, a professor emeritus in physics at the University of Alaska Geophysical Institute has been quoted by Dick Hutchinson as saying:

“These are interference fringes due to the parallel faces of the filter and to the narrow spectral emission at 5577 Angstroms in the aurora. That green, atomic oxygen emission line is the strongest emission in the aurora near our film and eye peak sensitivity, so it shows up first when there is any device in the optical path which sorts out the spectral emissions.”

You've been warned! I hope I never make that mistake again.

Best,

Chris