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Old May 8, 2011, 3:26 PM   #1
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Default Halo?

I've been asked to shoot some photos at a church. I took a couple of shots today, just get a preview of what the stage lights will do.

The photos are the same shot, one is a crop of the other.

My question: Around the speaker is something that looks like a halo. Some of the photos have the halo, some don't.

What causes the halo?

More important, how do I avoid it?

EXIF - f1.2 1/80 second ISO 640 85mm lens 18.1 meters from camera to subject.

Thanks
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Old May 8, 2011, 3:26 PM   #2
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Default crop

Here's the cropped shot.
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Old May 8, 2011, 3:52 PM   #3
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Looks like chromatic aberration, stop the lens down a little and it will probably go. Do a google on it for more info.
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Old May 8, 2011, 4:04 PM   #4
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^ What he said. Looks like maybe it is also combined with a tad bit of motion blur that exaggerates the aberration a bit.

bc
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Old May 8, 2011, 8:32 PM   #5
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First, I don't doubt the camera movement. I was hand holding and shooting from 20 meters. That will change when I shoot for real.

Second, how can I avoid C.A.?

Is it a simple as making a change in the settings. Mark says to step down. And Mark is a guru and I dare not disagree with him. On the other hand, I can mess up just about anything, so I'm looking for additional options in case Mark's advice is no match for my messing up ability.

Most of the stuff I've read so far, is about fixing it after it's happened. I'd rather prevent it.

Any ideas?

FP
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Old May 9, 2011, 8:00 AM   #6
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Quote:
Originally Posted by FaithfulPastor View Post
...
Most of the stuff I've read so far, is about fixing it after it's happened. I'd rather prevent it.

Any ideas?

FP
My 50mm f/1.4 does the same thing, when I posted this several years ago

This happens with wide aperture lenses on high-contrast scene like the one you're shooting. This CA varies with the angle, so try to shoot it from a different perspective so the light source does strike the lens from the same angle (other than closing the lens down):
http://community.the-digital-picture.../8/t/5011.aspx
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Old May 9, 2011, 8:49 AM   #7
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Quote:
Originally Posted by FaithfulPastor View Post
Second, how can I avoid C.A? Is it a simple as making a change in the settings. Mark says to step down. And Mark is a guru and I dare not disagree with him. On the other hand, I can mess up just about anything, so I'm looking for additional options in case Mark's advice is no match for my messing up ability. Most of the stuff I've read so far, is about fixing it after it's happened. I'd rather prevent it.


Wikipedia ( http://en.wikipedia.org/wiki/Chromatic_aberration ) has a good, succinct characterization of CA:
Quote:
Axial CA occurs throughout the image, and is reduced by stopping down (this increases depth of field, so though the different wavelength focus at different distances, they are still in acceptable focus). Transverse CA does not occur in the center, and increases towards the edge, but is not affected by stopping down.
In digital sensors, axial CA results in the red and blue planes being defocused (assuming that the green plane is in focus), which is relatively difficult to remedy in post-processing, while transverse CA results in the red, green, and blue planes being at different magnifications (magnification changing along radii, as in geometric distortion), and can be corrected by scaling the planes appropriately so they line up.
What Mark is saying is that your image shows LoCA (longitudinal CA -- another name for axial CA). The way someone like him knows that is that the CA is in the middle of the image and is a uniform purple around the person. Neither of these things is true of lateral CA (AKA transverse CA). LoCA is a property of a particular lens’ design and can be eliminated or reduced by lowering the aperture that you shoot at. If you examine the CA diagrams on, e.g., SLRGear.com, you will see that some lenses (and on many zoom lenses, some particular zooms) have a relatively high CA wide open that drops to a minimum when the aperture is stopped down a couple of stops. That is the characteristic signature of LoCA. The rest of the CA in the diagram is lateral CA -- which is not affected by aperture and is by far most prominent at the edges of photographs. Lateral CA is very easy to get rid of in post-processing. My Capture NX2 application will automatically remove lateral CA from images. Another really good thing about this kind of CA is that it doesn't lower the sharpness of the image when it is removed.

LoCA, however, cannot be auto-removed from photos with any PP software that I know, and removing it lowers the sharpness of the image. The ways to avoid or lower LoCA include:
1. Buy a lens that doesn’t suffer from LoCA;
2. Stop a LoCA-prone lens down a stop or two;
3. If you are using a zoom, know what zoom ranges of the lens are LoCA-prone and avoid those zooms;
4. Avoid high-contrast between your subject and the background in your composition with a LoCA-prone lens.
FWIW

Last edited by tclune; May 9, 2011 at 8:54 AM.
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Old May 9, 2011, 9:53 AM   #8
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I just fought my way thru an event last week trying to get good photos of speakers on stage. a couple good, mostly bad. changing light colours, intensity and direction all conspired. not in my favor.

i spoke with a pro who does this frequently and he uses "pop and drag". another solution i thought of but haven't tried is to go manual and wait for the light. then bracket. i don't know if that'll work better but i do know using aperture priority with spot metering sure didn't.

so the question is: can you use flash?
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Old May 9, 2011, 2:54 PM   #9
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I think setting your meter by spot metering off the shirt to bring down the exposure of that would help as well as stopping down. His shirt is definitely overexposed. BTW - the slight motion blur that I think I see along with the CA is probably caused by his slight torso movement rather than movement from the camera.

brad
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Old May 15, 2011, 1:20 PM   #10
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This looks more to me like the phenomenon called 'blooming', due to overexposure. The b/g is blue and the shirt is white and overexposed, so the interpolation of the edges has no intermediate value. As DigMe said, meter off the shirt. Or get the speaker to wear a different color, so the contrast isn't as high.
The chromatic aberration is due more to the Bayer filter than to the lens, and there are some Raw converters which can reduce it, but the best way is to avoid it in the first place.

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