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03-09-2015, 05:09 PM   #1
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Chromatic Aberration or Fringing?

Hi Everyone,

I was at Daytona Bike Week recently and took my K-5 II with the 16-50mm lens. It's not a lens that I don't usually use but I liked using it there and I really like the lens. Something I noticed about my photos after I took them. I have had a problem with a sort of purple or blue fringing associated with metal parts of motorcycles. It was gone when I adjusted my settings and took another photo. Here are some heavy crops:

The one with the fringing was taken at 1/160, ISO 200 at F/2.8. The second one at 1/60, ISO 500 at F/8. I think using F/8 was the contributor to losing the fringing. Any thoughts?

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03-09-2015, 05:31 PM   #2
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I think it's because photo is out of focus at f2.8. You probably shot at the close proximity of the bike (I see you used widest end zoom), so focus pane is very narrow, and out of focus metal reflection probably causes that fringe...
03-09-2015, 05:37 PM   #3
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That looks to be PF. There is one line of thinking that holds PF to be longitudinal CA in the UV portion of the spectrum re-emitted as a longer wavelength bloom at the sensor surface*. Stopping down increases the DOF and minimizes the effect.


Steve

* I was unable to find the reference, but the study was done using narrow band LED lamps where the spectrum was UV deficient. PF was absent until a UV component was added to the spectral mix.
03-09-2015, 05:41 PM   #4
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QuoteOriginally posted by stevebrot Quote
That looks to be PF. There is one line of thinking that holds PF to be longitudinal CA in the UV portion of the spectrum re-emitted as a longer wavelength bloom at the sensor surface*. Stopping down increases the DOF and minimizes the effect.


Steve

* I was unable to find the reference, but the study was done using narrow band LED lamps where the spectrum was UV deficient. PF was absent until a UV component was added to the spectral mix.
This is the reference I usually cite:
Digital Photography FAQ

---------- Post added 03-09-15 at 08:47 PM ----------

QuoteOriginally posted by traderdrew Quote
Hi Everyone,

I was at Daytona Bike Week recently and took my K-5 II with the 16-50mm lens. It's not a lens that I don't usually use but I liked using it there and I really like the lens. Something I noticed about my photos after I took them. I have had a problem with a sort of purple or blue fringing associated with metal parts of motorcycles. It was gone when I adjusted my settings and took another photo. Here are some heavy crops:

The one with the fringing was taken at 1/160, ISO 200 at F/2.8. The second one at 1/60, ISO 500 at F/8. I think using F/8 was the contributor to losing the fringing. Any thoughts?
I've used a lot of Pentax lenses and find them to be somewhat prone to purple fringing (which is a form of chromatic aberration) wide open. The newer lenses, those released in the last ~3 years or so, are better. Stopping down helps resolve it.

03-09-2015, 06:46 PM   #5
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QuoteOriginally posted by carpents Quote
This is the reference I usually cite:
Digital Photography FAQ
Cool! Thanks!


Steve
03-09-2015, 06:48 PM   #6
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QuoteOriginally posted by stevebrot Quote
That looks to be PF. There is one line of thinking that holds PF to be longitudinal CA in the UV portion of the spectrum re-emitted as a longer wavelength bloom at the sensor surface.
If that was the case the UV light would have to have an intensity nearly four times that of visible light for such a spectral shift to be visible on a digital sensor. I'm not sold on the Ultraviolet hypothesis, the reason being is that borosilicate glass is pretty damn good at attenuating those high frequencies, and also the existence of a UV/IR absorbtion layer in font of the sensor will also soak up residual UV light.
03-09-2015, 07:05 PM   #7
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QuoteOriginally posted by Digitalis Quote
I'm not sold on the Ultraviolet hypothesis, the reason being is that borosilicate glass is pretty damn good at attenuating those high frequencies
I understand what you are saying, but I believe it is the near UV light that is in question, plenty of which makes its way to the sensor. As you know, it is that portion of the spectrum that is so problematic for landscape film photography.

QuoteOriginally posted by Digitalis Quote
and also the existence of a UV/IR absorbtion layer in font of the sensor will also soak up residual UV light.
...and may be the cause of the problem.

I would have to do the experiment myself to see whether there is any truth to it, but the absence when UV is absent followed by presence when UV is present would seem to implicate UV.

As for intensity, I guess it would depend on the light.


Steve
03-09-2015, 07:13 PM   #8
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QuoteOriginally posted by stevebrot Quote
As for intensity, I guess it would depend on the light.
To expand a little, I have two lenses that have the reputation for PF, the FA 35/2 and FA 77/1.8. In general use, I seldom see PF with either lens. A few months ago I set off to compound a recipe to reliably create PF. This is how I do it:
  • Crumpled aluminum foil
  • Electronic flash as main light source
  • About two stops over-exposed
  • Aperture wider than f/2.8
  • Focus on near point of foil wad
  • Subject distance about 24 inches (~60cm)
The PF shows as malignant bloom on the highlight surfaces of the OOF areas. For this scenario the light is quite intense with many of the sensels fully clipped.


Steve

03-09-2015, 07:32 PM   #9
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Then you should measure the number of photons emitted by the flash with frequencies of 350-200nm with energies greater than say 3.10ev up to 6.2ev - As far as I'm aware, no one has ever quantified this. There are emission curves for Xenon arc flash tubes but we have no idea of the longevity of emission - it is possible the Ultraviolet pulse from flash is longer than the visible pulse, which would explain the increase of purple fringing with flash.
03-09-2015, 09:39 PM - 1 Like   #10
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In most cases when this fringing is seen at wider apertures but not when stopped down, it is the result of dispersion and spherochromatism. Colors are going to separate when refracted. The designer must prevent this by using glasses that bring the colors back to focus at the focal plane. However, not all colors can be be corrected and there will be a residual called the secondary spectrum ( in achromatic lenses) and tertiary spectrum ( in APO lenses). Even a highly corrected lens will have a sweet spot in the area of the optic where it is better corrected than in other areas. This is typically in the paraxial (central) and zonal areas (out 20- 60% from the optical axis)of the lens. The marginal zone is usually poorly corrected because the designer knows that the shooter can stop down the lens to truncate the poorly corrected marginal rays. With color fringing, what you are seeing is the uncorrected colors coming from the marginal zone of the lens. One color from different zones of the lens will not focus at the same point. This is the combination of Longitudinal Chromatic and spherical aberration (Spherochromatism).
03-10-2015, 07:52 AM   #11
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@traderdrew, processing software often has tools for fixing fringing. Lightroom lens correction is one way to do it.

My copy of the DA* 16-50 is underwhelming at f2.8. I only shoot f2.8 if it's very dark or I want minimal depth of field. Stopping down to f4.0 makes it a spectacular lens.
03-10-2015, 08:02 AM   #12
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If UV was the culprit, wouldn't adding a UV filter reduce or eliminate it? From everything I've read UV filters are basically only useful as damage protection for lenses on digital cameras.
03-10-2015, 09:44 AM   #13
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QuoteOriginally posted by Sagitta Quote
If UV was the culprit, wouldn't adding a UV filter reduce or eliminate it? From everything I've read UV filters are basically only useful as damage protection for lenses on digital cameras.
UV filters have different cut-offs, they don't perfectly start/stop at the visible wavelength.

I understand the same as you for utility of UV filters on digital - they are not sensitive to UV like film is, which is why the filters are less useful. But they're obviously not 100% insensitive to it because people mod their cameras to shoot exclusively UV.
03-10-2015, 10:33 AM   #14
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The K-5ii does not seem to have any great problem shooting a UV LED - I could probably get a better shot if I could be ars*d to set it up properly
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03-10-2015, 10:36 AM   #15
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Man this post is getting so scientific!! To sum it up, it sucks that the lens has such bad fringing in the OOF areas. Also, the bike isn't in good focus, causing the fringing to be all over your subject! The second image also was not in good focus, eliminating the fringing, but preventing you from creating a quality photo.

All this talk about science experiments and adding filters is pretty much a waste of time. Try focus peaking or zooming in on live view to get that sharp focus and minimize that fringing!
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