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PostPosted: Fri May 23, 2008 7:03 pm 
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Ever wondered what "longitudinal chromatic aberations" (or short: longitudinal CA) is and how it looks like?
Now, here is a typical example:

Image

You can easily see, what the problem is: in front of the point of sharpest focus things get a violet tint, behind a greenish coloration. This is also sometimes called "Bokeh CA" as the out-of-focus areas are affected. This sort of CA is very hard to correct, as no programm can identify whether a given unsharp pixel-cluster is in front or behind focus.
This is in stark contrast to lateral CA which produces a colored fringe of a certain width (typically1-2 pixels) to one side of the original high-contrast border and can be quite easily corrected in-camera or with post-processing.

Not always is this effect distracting, as you can see in the following shot. The bokeh CA can be seen as a nice green "halo" around the head of the dandelion.

Image

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PostPosted: Fri May 23, 2008 7:07 pm 
I follow you,

so how does one achieve this?


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PostPosted: Fri May 23, 2008 7:15 pm 
alex168 wrote:
so how does one achieve this?


Actually, I think we're supposed to avoid chromatic aberrations...


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PostPosted: Fri May 23, 2008 9:33 pm 
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Yeah, CAs - whether longitudinal or lateral are both "aberrations" and as such not desirable.
Unfortunately, lens design is a very complex task and it is a fact of life that different wave-length (aka "colors") have different refractive values in lenses. So it's not easy to correct for all of the potential pitfalls in building a lens at a certain price-point!
So, unless your are owner of a lens with longitudinal CAs, there is not much you can do about it...
...unless you own a Nikon DSLR and buy a Nikon 180/2.8 :wink:

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PostPosted: Fri May 23, 2008 10:11 pm 
Is there any websites or books/articles on how to either remove chromatic abberation or at least avoid it in the first place?


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PostPosted: Fri May 23, 2008 10:24 pm 
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There's nothing you can do, Welly, except from
(a) buying a lens without this "error"
(b) stopping the lens down
(c) not shooting dark fine/slim objects against a glaring sky
-------
As to lateral CAs most software (like e.g. CaptureNX) offers some kind of treatment against it...

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