tom hudson Posted November 12, 2004 Share Posted November 12, 2004 I have an AF Nikkor 28-85, f/3.5-4.5 lens that I've had for a while. I took some photos of the aurora borealis a couple of days ago with my D100 and the images all have a strange circular artifact at the center. Looks like some sort of light interference pattern (see image at http://klanky.com/images/LensArtifact.jpg). The first time I saw this, coincidentally, was an aurora image I shot with this same lens, but on a 6006 body shooting film. I had the image put on a Photo CD and when I looked at it, I figured the Photo CD guys had screwed it up. Apparently not! Anybody seen anything like this before? Any ideas what the cause might be? Suggestions appreciated!<div></div> Link to comment Share on other sites More sharing options...
streetlevel Posted November 12, 2004 Share Posted November 12, 2004 Do you have any filter on this lens? I'm guessing not, but it's worth a shot... It looks to me like an internal diffraction pattern from something on the interior of the lens that would be projecting rearward (in the retrofocus area mebbe?) Definitely odd...I've never seen that before. Link to comment Share on other sites More sharing options...
frank uhlig Posted November 12, 2004 Share Posted November 12, 2004 On a computer screen these interference rings are called Moire ... or such; there are ways on my monitor to elimnate these, so maybe this is a digital, sensor, file compression Jpeg like artifact. ???? Link to comment Share on other sites More sharing options...
aardvarko Posted November 12, 2004 Share Posted November 12, 2004 I've seen this pattern with a Coolpix; it is not present with my D70. I'd imagine a lens defect. Link to comment Share on other sites More sharing options...
tristanlaing Posted November 12, 2004 Share Posted November 12, 2004 I have the same lens (love it), and I havn't had the same issue. Lots of night photography with it, no artifacts. Does it show up in any other pictures you've taken? Link to comment Share on other sites More sharing options...
wogears Posted November 12, 2004 Share Posted November 12, 2004 Looks like Newton's Rings to me. Could be from an element that is separating. I assume this does NOT happen with other lenses? Look through your lens with a strong light, and see if you see anything. Les Link to comment Share on other sites More sharing options...
darren_cokin Posted November 12, 2004 Share Posted November 12, 2004 I have never seen an Aurora, but I do spend a lot of time at the office with nothing to do but browse the internet. Your question reminded me of something I read a few days ago. I don't know if this was the original page I read this, but it's along the same lines. Here is a clip from http://www.keweenawimage.com/auroraphoto.html Begin pasted text: With my Nikon lenses I have found that long exposures result in concentric circles showing up in the middle of the images when I use a filter of any kind. Nikon says this is due to the high reflectivity of the aurora. Thanks to the University of Alaska forecaster, the explanation follows. "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." So, don't use filters! Link to comment Share on other sites More sharing options...
alex_lofquist Posted November 12, 2004 Share Posted November 12, 2004 Maybe Newton's rings. But where? If it were a lens separation, it should not focus the pattern at the focal plane. Generally Newton's rings are not nearly so circular. The light source must be monochromatic (single color), otherwise you would have "rainbow" effects here. Some aurorae are almost monochromatic so I would look to see if the interference patterns are only on those pics. Also, is the pattern always centered on the frame? On the film shots are the rings on the negative? If not, wouldn't rule out the lab screwing it up. Link to comment Share on other sites More sharing options...
alex_lofquist Posted November 12, 2004 Share Posted November 12, 2004 I think that Darrin has the right answer if you had an interference filter on the lens. Link to comment Share on other sites More sharing options...
tom hudson Posted November 12, 2004 Author Share Posted November 12, 2004 @Will: I have a Tiffen Sky-1A on there. I'll try a shot or two with that off and see what I get. @Frank: I would have thought so, but the same thing showed up on a film exposure on my 6006. At least, on the Photo CD version of it. I'll see if I can dig up the negative on that and verify that it's actually on the film. @Tristan: I've only seen it in low-light, long exposure images (two different aurora borealis shots, by coincidence). Both were at the 28mm end of the operational range. @Les: I took the filter off and looked through the lens at a bright light, there was nothing obvious visible. However, looking at some photos on the Web regarding Newton's rings, which I hadn't heard of before, it sure does look like that may be what's going on. I wonder if two of the elements are touching at the wide-angle end of the lens's range, causing the rings. I will run some tests ASAP. THANK YOU for all the responses; I'll report back after making some test exposures. Link to comment Share on other sites More sharing options...
alex_lofquist Posted November 12, 2004 Share Posted November 12, 2004 Sorry, that should be Darren! Link to comment Share on other sites More sharing options...
tom hudson Posted November 12, 2004 Author Share Posted November 12, 2004 Good heavens! You guys answer so quickly I had three more responses by the time I finished composing my response! I think Darren has nailed it -- the ONLY time I've seen these rings is on aurora photos, and I have the Sky-1A filter on all the time to protect the lens (D'OH!!!) Too bad there's no aurora activity tonight; I'd love to get out and test the theory. I will do some general long exposures and see if anything turns up with no aurora in the scene. Link to comment Share on other sites More sharing options...
streetlevel Posted November 12, 2004 Share Posted November 12, 2004 Looks like your aurora was brighter than ours here in Chicago...but then we had to compete with the downtown glow... Got any other pics up we can gawk at? ;) Link to comment Share on other sites More sharing options...
conrad_hoffman Posted November 13, 2004 Share Posted November 13, 2004 I think Darren has nailed it. That's the classic interference pattern produced by a plane parallel interferometer (Fabry-Perot) combined with a lens. You need a monochromatic light source to see it, such as the narrowband emission from an aurora! Link to comment Share on other sites More sharing options...
conrad_hoffman Posted November 13, 2004 Share Posted November 13, 2004 BTW, in laser systems it's common to have filters and windows specified with a small "wedge" angle between the surfaces. This causes the interference fringes to be so close together so as not to be an issue. Link to comment Share on other sites More sharing options...
tom hudson Posted November 13, 2004 Author Share Posted November 13, 2004 OK, I went out and shot some 10-second exposures of the night sky, with and without the filter, and with the lens at 28mm and a bit higher, to test the theory that the optics were touching at 28mm or something else was out of whack. I downloaded the images and cranked up the contrast to see if there was evidence of the artifacts. There was none on any of the exposures. Soooo.... I'm going to keep my eye out for northern lights and the next time they show up, I'll have the filter off the lens. In fact, I've taken it off and will leave it off. I was told years ago with my first SLR to get a Sky-1A filter, put it on and leave it on to protect the lens. I wish I hadn't been told that. When I get more photos, I'll report back and post them. I _REALLY_ appreciate the quick responses from all of you. I just found this forum today but will be joining up. @Will: Go to http://astronomy.klanky.com, I've slapped a few images up there for your amusement. I have to go dig through my archives for more, and when I get some free time I'll add them with some more descriptions. You'll see the circular artifact on the two aurora images. Link to comment Share on other sites More sharing options...
jim_pipkin1 Posted November 15, 2004 Share Posted November 15, 2004 About 15 years ago, I found a similiar "artifact" on one of my aurora photos (taken with my Nikon FM2 and a 50mm f1.8 lens). At first I thought it was a printing error, but it appeared on my negative too. I thought it was the strangest thing at the time, especially since I had taken a lot of star trail photos with the same set-up and never had a pattern like that show up. Good to know it has happened to other people too. Next time I get to shoot auroras, I'll try it with and without the UV filter. Link to comment Share on other sites More sharing options...
wogears Posted November 16, 2004 Share Posted November 16, 2004 Newton's Rings are one specific example of an interference pattern. I think Darren got it exactly right, but I had the general idea... :-) Les Link to comment Share on other sites More sharing options...
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