Strange pattern on long exposures

[gekko]

I was doing some night time photograhpy of the northern light the other day, using exposures of 5-20 seconds. In all the images I see this circular riple pattern in the dead center of the images, see attached image (which is a crop of the centre of the original image).

I haven't done any long exposures at night before, so I haven't noticed this previously. Any idea of what it is caused by?

All pictures were taken in RAW with my 7D and 15-85 lens (see signature) on a tripod, exposures of 5-20 seconds at 15 mm, f/3.5, ISO 400-800.

 

[lintoni]

Do you have a filter on the front of the lens? I think I've seen something similar on a photo on the web and it was attributed to a filter.

If not, do you get the pattern with other lenses?

 

[gekko]

Thanks for your answer.

I did have a Hoya UV filter on. That struck me as well as a possible source. I tried this evening with the exact same setup, minus the northern light. I was not able to replicate the issue. Tonight I cannot see any difference when comparing images shot with/without the UV filter, i.e. the circular pattern is not present in my shots tonight.

Could it be related to the northern light? Or maybe the angle of the northern light? I don't know, but it seems like I have to wait until the next northern light night before finding out.

 

[Loswr]

As you found, it was the filter. For more information on the phenomenon, google "Newton's rings".

 

[gekko]

Thanks for the reference!
Although the physics ruined my shots, it's very interesting to read the theory on why it happened. We live and learn. Now I know to remove the filter next time the northern light is in my viewfinder.

 

[Tanispyre]

Newton ring interference is dependent on both the reflective surfaces, and the frequency of the light passing through. Light from the Auroras is fairly monochromatic, and may have been just the right frequency to show as light and dark Newton rings, rather than the rainbows seen by white light. It is just a guess but it could be an explanation on why you were unable to reproduce the effect the following nights with and without the filter.

 

[Mt Spokane Photography]

Lens makers use Newtons rings to fine tune grinding of lenses. They are far more sensitive than the best direct measuring tools. I've had the same issue with measuring tolerances in certain parts, they need tighter tolerances than is possible to measure, so clever people have found indirect ways to check the accuracy, and to create "Gold" standards where measuring is impossible.

 

[gekko]

Newton ring interference is dependent on both the reflective surfaces, and the frequency of the light passing through. Light from the Auroras is fairly monochromatic, and may have been just the right frequency to show as light and dark Newton rings, rather than the rainbows seen by white light. It is just a guess but it could be an explanation on why you were unable to reproduce the effect the following nights with and without the filter.

That makes a lot of sense. Thank you!