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DPChallenge Forums >> Photography Discussion >> Weird circles in a photo
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01/31/2005 06:51:10 PM · #1
I just noticed something strange in one of my recent photos:

nljosahringir.jpg

This is a downsized 100% crop of an aurora, taken with a 17-40 f/4L at f/5 for 25sec. It was shot in a hurry, so I had a UV filter on it and some dust, but I haven't seen anything like this before though.

Is it my UV's fault or am I receiving messages from outer space all of a sudden?
01/31/2005 06:56:53 PM · #2
difficult to tell without a larger image.
I'd guess focus not right or effect of long exp time i.e. 25 sec.
01/31/2005 06:59:06 PM · #3
Most likely the UV filter. What kind is it?
01/31/2005 07:04:06 PM · #4
I get dots like this most of the time with any low light shooting, but I don't have a DSLR. It really hurts in basic editing when you can't remove the spots. I had always assumed it was part of the noise issue.

Message edited by author 2005-01-31 19:04:39.
01/31/2005 07:04:21 PM · #5
I've seen similar rings in my NL photos but w/o the UV filter. Not on the 17-40 though, but on my 50 1.4 at 1.4 at long exp.
Could be lens dust, stars starting to blur on long exp (esp if focus is not right?), sensor dust?
01/31/2005 07:08:31 PM · #6
Maybe a fingerprint?
01/31/2005 07:15:49 PM · #7
see also this thread
related thread

i saw the same thing, under similar circumstances

142139.jpg
01/31/2005 07:19:19 PM · #8
0966516907.01.LZZZZZZZ.jpg

It's the aliens!!!!
01/31/2005 07:20:02 PM · #9
i might be in Dover, Sunday


01/31/2005 07:26:19 PM · #10
Example:
134115.jpg
01/31/2005 07:49:45 PM · #11
The phenomenon is referred to as "Newton's rings". It's an interference pattern caused by two surfaces that are almost, but not quite, in contact. In this case, it looks like two surfaces are touching at the center of the frame.
The next question is WHICH two surfaces? Since the rings are visible at the sensor plane, the plane where they are created must be either in focus (or nearly so) or be very near the sensor. I suspect that they are near the sensor, and are generated when cooling the camera results in warpage of the filter or sensor due to the mismatch in CTE (Coefficient of Thermal Expansion).
01/31/2005 07:52:28 PM · #12
Originally posted by kirbic:

The phenomenon is referred to as "Newton's rings". It's an interference pattern caused by two surfaces that are almost, but not quite, in contact. In this case, it looks like two surfaces are touching at the center of the frame.
The next question is WHICH two surfaces? Since the rings are visible at the sensor plane, the plane where they are created must be either in focus (or nearly so) or be very near the sensor. I suspect that they are near the sensor, and are generated when cooling the camera results in warpage of the filter or sensor due to the mismatch in CTE (Coefficient of Thermal Expansion).

In that case these rings should form when a lens is cooling down to the ambient temperature and then disappear. Right? Or at least disappear when everything has cooled down to same temp and CTE removed from the eq. Right?

Edit: I, of course, mean camera to cool down to local temp and all the layers of sensor to be of same temp and CTE neutralised.

Edit 2: I've never experienced these rings/blobs with my 17-40 but often with 50 1.4 at max apperture (i.e. 1.4) and then not when I change lenses to 17-40 (in a timeframe that would make temp adjustment unlikely)

Edit 3: my gut feeling, is that this is a result of not focusing at inf. but slightly shorter, so this is in effect bokeh of stars. But, I could easily be very wrong.

Message edited by author 2005-01-31 19:57:22.
01/31/2005 08:11:11 PM · #13
Couple comments...
First, while it is possible that the effect is transient and would disappear after the lens/camera equilibrates at the outside temperature, it is also possible, and in fact probable, that it wouldl not be a transient effect. Every substance changes dimension a little bit with every degree heated or cooled. The silicon sensor will change by about 4 ppm per degree C of temperature change, for example. When two objects of dissimilar material (and CTE) are bound together, but undergo a change in temperature, the result will be warpage of the whole assembly. The warpage will stay as long as the objects remain at that temperature, assuming there's no "creep" going on in the system.
Second, I don't think these have anything to do with OOF stars. Where you are focussed, they would only be slightly out of focus, and should appear as small, round dots.
01/31/2005 08:16:59 PM · #14
kirbic is correct, they are Newton rings. To see these rings the light needs to be pretty much one wavelength, not something that you normally run into. My guess is that the green in the aurora is pretty monochromatic and therefore shows the rings much more then normal.
01/31/2005 08:19:08 PM · #15
cool - so it is a phenomenon, not solely created by the camera & temp...
but actually by light.


01/31/2005 08:30:03 PM · #16
It is both the lens/filters and the light, not just any light will show these rings.
01/31/2005 08:33:24 PM · #17
These circles, (Newton's Rings) usually are most visible at the interface between two optical elements in monochromatic light. I would guess that these particular ones are a result of the sensor's protective cover proximity to the sensing element, hence the uniform and centered appearance. I'll bet you would not see these with short exposures because of their low contrast with less than perfectly monochromatic light.
The curved streaks of light are star trails caused by the rotation of the earth.
02/01/2005 03:23:01 PM · #18
Newton's rings? Definitely has the sound of rare phenomena about it :)

The picture was taken on my doorstep, and the camera had only been out in the cold for a couple of minutes so this thermal expansion thing sounds right. You had similar conditions, Gauti and soup?

Thanks for all the replies people, I can stop worrying about aliens now.
02/01/2005 03:44:11 PM · #19
the similarity is the long exposure, the light source, and the resulting rings ;}

edit: oh yeah, and the camera make...



Message edited by author 2005-02-01 15:44:45.
02/01/2005 03:54:50 PM · #20
On his webpage at http://www.ptialaska.net/~hutch/aurora.html Dick Hutchinson discusses this, and quotes a University of Alaska forecaster, Chuck Deehr 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."

02/01/2005 03:55:50 PM · #21
It almost looks like moire, except the pattern is circular.
02/01/2005 04:32:09 PM · #22
Are you sure it isn't a warp in the space/time continuum, captain?
02/01/2005 05:16:02 PM · #23
I took a photoshoot in November during an aurora borealis and noticed these little suckers. Took me a few minutes to figure out what the hell they were, but I think my explanation is very plausible.


Every good lens has a multitude of anti-reflection coatings on it, but each coating can only be tuned to a particular wavelength. What I assumed was that they made these coating with a variable thickness as a function of distance from the center to used then as "multiwavelength" coatings so that they can respond to broadband light better.
Now, Aurora borealis creates a monochromatic light related to the specific energy level transition of the electron. In this case, the light is green and has a precise wavelength (as precise as Heisenberg's uncertainty principle allows it to be, which is very precise) of about 557.7nm. So, for a monochromatic light source (which is very unusual in photography) the coating responds with an interference patterns which corresponds to the distribution of the thickness of the coating. That's why the aurora alternatively interferes constructively and destructively as you move away from the center. I don't think the cold changes anything as I was outside for about 3 hours before taking my pictures.

Here are my Aurora Pictures. Taken from an abandoned building in downtown Montreal. Until then I would have never imagined there could be an aurora in DOWNTOWN montreal.

http://www.imagestation.com/album/?id=2134506440

Here's an example of the rings. they always appear deadcenter on the picture

142489.jpg
02/01/2005 05:29:18 PM · #24
I encountered the same problem shooting aurora this winter. I was using my 20D with a 50mm f/1.8 mii with a canon UV filter (wide open). Labuda's theory sounds great, but things are getting very technical here, hehe. Had no problem with my kit lens in same conditions. (went out with the kit lens on camera, everything cooled down and then went inside for my 50mm which wasn't as cold...)
02/01/2005 05:47:54 PM · #25
I believe those are shockwave rings from a UFO.
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