Bad blue fringing from nowhere.

December 31, 2008 2:48 AM

What might be causing the really bad (never before seen) blue-fringing around the guy on the left's shirt in this photo my wife took?

This photo was taken on our Olympus E-500 DSLR in RAW mode, 14-50mm Zuiko lens, polarizer filter, with some simple exposure adjustment as the only post-processing. Other photos of this group of people have the same problem around this guy's shirt, but we've never seen any blue-fringing (common with point-and-shoot digital cameras) with this camera before, around anyone or anything, in the last 2 years / 60,000-odd photos. No visible fingerprints on the filter or lens.

What's the verdict - chromatic aberration from the lens / polarizer? Some encroaching problem with our camera? We've captured an aura? Or just bad luck?

posted by Jimbob to media & arts (10 answers total)

Wow. This is a good one.

Blue fringing is chromatic aberration. It seems to be strongest in the +radial direction (outwards from the detector center), and where the white values are strongest, with the darkest background.

For instance, it's most prominent around the upper-left shoulder (saturated white against near-black), whereas the lower-right sleeve barely displays any at all (which is much closer to the center of the picture).

Chromatic aberration typically results from a color-dependent focal length: blue focuses nearer (or farther) than green, and red focuses on the other side of green. If the red->green focus points are relatively close, only the blue portion of the spectrum will display the chromatic effects - and so you end up with blue banding.

So, here's a test: cut some small white squares of paper, and photograph them near-saturation (or slightly saturated) spread against a black background. If you can't find anything else, pieces of white tape on a window pane at night will do. If the squares at the outer edges of the photo show blue banding, and the ones in the center do not, I'd say it's pretty clear what's going on (and maybe you've just been lucky in contrast placement, or never noticed this before).

Obviously, the next step is to see if removing the polarizer has any effect, although I doubt it's the culprit, as it doesn't contribute optical power.
posted by IAmBroom at 3:47 AM on December 31, 2008

It looks to me like the actual point of focus is on the background fauna, as opposed to being on the human subjects' faces.

I'd say that this is the problem here.

Also, with digital photography and most particularly with zoom lenses with many glass elements, I'd stay away from using any kind of filters on the front unless absolutely necessary.

Four more air/glass surfaces from that polarizer are not something you want or need with a shot like this one.
posted by imjustsaying at 3:55 AM on December 31, 2008

...uh, that would be foliage, not fauna in my answer above.
posted by imjustsaying at 4:00 AM on December 31, 2008

I had a lens a few years back, basic zoom, that was really good for my nieces and nephews' sporting events. No fringing, but then again the teams' colors were black and gold. Then I tried to use the lens for some water lily ponds in bring sunlight. The fringing was so bad i only kept the shots as an example of really bad fringing. In that case, though, the colors were mutant green and bright yellow-white.

Point being, sometimes a certain color combo comes up that displays fringing more than usual.
posted by notsnot at 6:16 AM on December 31, 2008

His shirt might have been washed in one of those detergents that make things glow in the dark. You can't see it in daylight, but it might have confused the camera.
posted by gjc at 7:28 AM on December 31, 2008

His shirt might have been washed in one of those detergents that make things glow in the dark. You can't see it in daylight, but it might have confused the camera.

That was my first thought, too. UV light is even worse for chromatic aberration than blue light is, because it is that much further up the spectrum. Did you take your UV filter off when you slapped the polarizer on?
posted by ikkyu2 at 8:05 AM on December 31, 2008

Just get rid of it in your RAW converter. It's easy to fix in ACR.

Also, it's not just on the guy's shirt, it's heavily on the edge of the trees in the top left, though that's less easily noticed. Unless you're printing this photo at a large size, it's pretty ignorable on the shirt.
posted by Magnakai at 10:17 AM on December 31, 2008

I would agree with IAmBroom. I also want to add that the beige sweater is adorable.
posted by Foam Pants at 1:18 PM on December 31, 2008

As many have said it's chromatic aberration. For future photos stopping down will reduce/prevent it. You'll find it appears most terribly in areas of stark contrast changes when you are wide open or near to it.

It can effectively be removed in photoshop RAW settings by tweaking settings in the lens correction tab. Later versions of photoshop even have a special chromatic aberration and defringe slider. If you don't have the RAW or a recent version of photoshop or can't get it all that way it can be done in photoshop manually. Googling will find lots of step by step directions but if you know photoshop just create an adjustment layer that is desaturated for that particular color and paint the worst away so that it effectively blends with the white shirt.
posted by tinamonster at 8:49 PM on December 31, 2008

Well, the problem led me to discover the fringing slider in Lightroom, which does a pretty reasonable job. I'll do the tests IAmBroom suggests, and consider some better optics. Although this lens has served us well - it's the first photo where we've ever noticed the problem.
posted by Jimbob at 9:46 PM on December 31, 2008