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05-21-2012, 12:36 PM   #1
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What does CA look like in the real world...

I've just been through some (and stupidly deleted) photos from this weekend and quite a few from the FA35 appeared to have what I think was fairly heavy CA...

Shots with trees in them especially seemed affected... Almost like the branches had a kind of purplish-glow... Is this CA?

Never noticed this kind of thing with any of my other lenses and it's not a deal breaker on the FA35(for now) as I'm really enjoying using it... But wonder if CA is a known issue of this lens(?) as I don't remember it being mentioned in the user-reviews section...

Thoughts?

05-21-2012, 12:44 PM   #2
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The usual name for this seems to be "purple fringing."
Klaus has an egregious example at the bottom of his page
Pentax SMC-FA 35mm f/2 AL - Review / Test Report - Analysis
05-21-2012, 01:51 PM   #3
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The wikipedia entry has a couple of example photos, and also a link to a page about lens flare which is an issue I seem to have hit with my SuperTak recently.

Chromatic aberration - Wikipedia, the free encyclopedia
05-21-2012, 04:02 PM   #4
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CA is one of two types of specific optical aberration and can be measured. PF (also known as color bloom) is a poorly understood interaction between lens and sensor that is similar in appearance to CA. CA can be readily demonstrated on film, while PF simply does not exist on that medium.

What you are describing sounds like PF. It appears as a bright blue to purplish fringe that often "runs" between adjacent clean borders. The typical case for PF is bare tree branches against a bright sky or specular highlights on water or brightly lit crinkled cellophane. The image linked above from photozone.de is a good example, though I must point out that I own the FA 35/2 and have never seen such flagrant PF in my photos with that lens

CA occurs when different color light converges at different points at the film plane. It shows itself with a colored boundary at a high contrast border, say a backlit branch. The branch will be bluish on one side and yellowish on the other. (Red/cyan is the other variant.) While most people are alarmed by the false color, the larger issue is a general degradation of sharpness across the frame. Lenses with significant CA are not sharp.

Here is a full resolution crop of a shot with my Vivitar 135/2.8 (Komine) on my K10D that shows CA...




Steve


Last edited by stevebrot; 05-21-2012 at 04:47 PM.
05-21-2012, 04:44 PM   #5
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Here is a better example of classic CA from a film negative from my Olympus XA.

First the full frame:




Looks pretty good until we pixel peep...



The Zuiko 35/2.8 on the XA is an impressive optic, but not in terms of CA. See the blue with matching yellowish tint in the rope itself? This looks a lot like PF, but this cleans up nicely in Lightroom using the CA correction sliders while PF will not.

Here is a reddish CA from the same frame...



Note that there is no obvious matching complement color on the other side. I suspect that this is a combination of multiple aberrations and not simple magenta/cyan CA. My reasoning is complex and not worth expounding on, but the reddish orange is pretty hard to miss. This too cleans up nicely in Lightroom.


Steve

Last edited by stevebrot; 05-23-2012 at 12:26 PM.
05-21-2012, 06:59 PM   #6
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When I read the title of the thread, I thought you should check the posts from @jgredline in the street shots thread for real world CA shots!!
05-21-2012, 07:07 PM   #7
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What I loosely call CA but is really PF... the A* 600.

05-21-2012, 09:59 PM   #8
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QuoteOriginally posted by littledrawe Quote
When I read the title of the thread, I thought you should check the posts from @jgredline in the street shots thread for real world CA shots!!
CA as in CAlifornia!!!


http://www.flickr.com/photos/jgredline/with/7231615326/


Steve

05-22-2012, 04:24 AM   #9
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This is true lateral chromatic aberration (300% crop to really make it obvious):



Notice on the left side of all the objects there is a magenta fringe, while the right side has a cyan fringe. As the various wavelengths of light are refracted differently, they don't line up, so some colors will extend out on one side, while the others appear on the other side. If it's the same color on all sides, it's not true CA.

Purple fringing is a phenomenon that seems to be specific to digital sensors. It is a purple halo on very high contrast edges, no matter which side, and there is no corresponding "opposite" color. Large aperture lenses are pretty susceptible to it.

Stopping down usually gets rid of PF (and longitudinal CA, a.k.a. "bokeh fringing"). Stopping down usually doesn't do much for lateral CA, and in some cases actually makes it worse. Lateral CA is relatively easy to correct in post-processing, PF not so much.

Last edited by Cannikin; 05-22-2012 at 04:35 AM.
05-22-2012, 06:47 AM   #10
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Cannikin has a good example of lateral CA where the image is impacted in the focus plane.

This is easily corrected in post processing if your image processor has a CA correction tool that rescales geometrically the three color layers.

I ran a thread on this type of correction some time ago

https://www.pentaxforums.com/forums/digital-processing-software-printing/1464...-software.html

There is also longitudinal CA which has the three colors focusing at different points, i.e. in front of or behind the plane of focus. This can be seen by a halo around objects either in front of the focus plane or behind it, usually purple in front and green behind the focus plane. Some people confuse this longitudinal CA with purple fringing, but it is clearly different.

CA in the out of focus regions is much harder to correct because it cannot be done geometrically.

WHen I ran the thread on CA correction tools, i looked at two basic types, ones specifically targeting lateral CA in the focus plane, and one that would be a fringe removal tool. For true lateral CA geometric rescaling can lead to excellent images, and greatly improved image sharpness off center, Fringing tools, if advanced enough can be vert good for removing longitudinal CA, Look for a fringe removal tool that lets you select the color casts to remove, as well as the radius in pixels, to remove.
05-22-2012, 06:57 AM   #11
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QuoteOriginally posted by Cannikin Quote
This is true lateral chromatic aberration (300% crop to really make it obvious):



Notice on the left side of all the objects there is a magenta fringe, while the right side has a cyan fringe. As the various wavelengths of light are refracted differently, they don't line up, so some colors will extend out on one side, while the others appear on the other side. If it's the same color on all sides, it's not true CA.
Nice example.


Steve
05-22-2012, 01:48 PM   #12
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QuoteOriginally posted by ElJamoquio Quote
What I loosely call CA but is really PF
This is the kind of thing I getting sometimes with the FA35... Is this PF or CA? What does PF stand for?
05-22-2012, 01:51 PM - 1 Like   #13
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QuoteOriginally posted by DaveHolmes Quote
What does PF stand for?
Pentax Forums.

Or purple fringing.
05-22-2012, 01:53 PM   #14
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QuoteOriginally posted by DaveHolmes Quote
This is the kind of thing I getting sometimes with the FA35... Is this PF or CA?
What ElJamoquio showed is an example of purple fringing.
05-22-2012, 10:08 PM   #15
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great thread and info. guys! I'd love to see a series of these where threads are created to show off the various optical properties good or bad. Things like microcontrast, curvature of field, decentered lenses. People could post shots of the various properties with real world examples full size and 100% crops would be very informative.
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