Road Train !!!!!!!!!!

by Russell McMahon

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I have looked on my new 50mm lens photo (I have Nikon D90) and I see "purple fringing" around the unfocused objects (longitudinal chromatic aberration). What's is going on? Is it common for this lens (Nikon 50mm f/1.4D) or I did something wrong?

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more detail:

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2 Answers 2

up vote 14 down vote accepted

What you're seeing is longitudinal chromatic aberration (otherwise known as axial colour), whereby the light at the edge of the bokeh disc undergoes a colour shift depending on whether it is in front of or behind plane of focus. The reason for this is that light of different wavelengths focus at different distances along the axis of the lens.

This is very common of ultrafast lenses like your 50 f/1.4 and is exacerbated by strong contrasts such as shooting against the open sky. Unlike Lateral Chromatic Aberration (where colours are shifted outward regardless of depth) which can be fixed easily in software, occurrence of Longitudinal Chromatic aberration depends on depth information, which is absent from the image. As such it is more difficult to correct automatically (though some software such as Capture NX2 and DXO, Lightroom 4+ offer this feature). It does however reduce rapidly on stopping down.

In a nutshell there's nothing wrong with the lens, it's just this type of subject/shooting environment is not ideal for f/1.4 lenses.

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As an aside there are ways to fix this with Capture NX2 automatically and Lightroom/Camera Raw. But the main thing is right, this is normal for this situation and lens. –  nwcs Mar 22 '12 at 17:10
    
@nwcs Are you referring to lateral chromatic aberration? As far as I know Lightroom/ACR doesn't offer longitudinal ca correction. –  Matt Grum Mar 22 '12 at 17:30
    
My understanding is that with the lens profiles that they will also correct some measure of longitudinal aberrations in Lightroom 4. I should have been more clear that it was not a certainty. But it is a certainty for NX2. –  nwcs Mar 22 '12 at 18:20
    
I have tried DXO, Capture NX2. They don't help me to fix this bug. :( –  garik Mar 23 '12 at 5:53
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@PicturePerformer ah yes the Defringe option, which attempts to remove any colour fringing, be it longitudinal CA, lateral CA or purple fringing. I turned it off for all images when I found it was turning the leaves of my trees grey. Would probably work for the image in question though! –  Matt Grum Jun 27 at 13:10

I just started noticing this issue on my 1.4 lens as well.

I've been using this guy's method to fix it, since Camera Raw doesn't do much on this type of aberration:

http://www.youtube.com/watch?v=XOIz8irPuGo

Basically it involves making a copy of your background layer, blurring the heck out of it, switching it to a Color layer, applying a layer mask to hide it, then manually painting white on the mask over the fringed areas, which essentially desaturates the edges to eliminate the aberration. It works well in most of the situations where I've used it, you just have to be careful around areas that are meant to be saturated.

It sucks, though, since the whole reason I got this lens was to shoot wide-open with insanely shallow DOF.

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