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16/06/2012

Correcting Lensbaby’s 0,42x chromatic aberration in the GIMP

I use the Lensbaby kit a lot. I love it. It gives an ethereal look to the images, and actually, I even love manual focusing, and the fact that I have to change the aperture disks also by hand (no, I don’t have neither Sweet35 nor Edge80).

The thing is that LB is a 50mm fixed lens, and in an APS-C camera, this is a little bit too long to be a walkaround lens.

So, this is why I bought the 0,6x and 1,6x kit and the 0,42x adapter.

The 0,6x screws in front of the LB, and converts the 50mm lens into a 35mm lens (more or less):

The 0,42x works in a similar manner, but converts 50mm to 21mm. That’s wide, and I tend to prefer this length for my shots:

The problem with 0,42x is that it introduces a really big amount of chromatic aberration. So much that is really visible, even for someone who has no idea of what chromatic aberration is:

Original Detail Detail

…it shows the typical (albeit very extreme in this case) chromatic aberration problem: it increases from the center to the corners, where is more visible. Technically (I think), the different light frequencies -i.e. colours- are not focused in the same point, so the red, green and blue parts of the image are not totally aligned.

This may be what you want, and in some cases, it’s even desirable. As I said, Lensbaby is about ethereal, non-previsible images.
In most cases, though, this is a real problem. You can solve it, or at least try to correct it as much as possible, using the GIMP.
You can use the Chromatic Aberration operator, in the Colours menu:

I’m using the values you see here, but you’ll have to find the values you need for your camera, since this is resolution dependant. I’m using Lateral chromatic aberration correction. Blue: 10, Red: −10.
And I have to run this 2 or 3 times.

Here go the results:

Original After

The aberration is not completely solved, but at least is less visible. We still have a problem with the chromatic aberration caused by the camera itself (visible for example in places where there is a lot of contrast, for example in the leafs of the tree) and that has been magnified by the 0,42x.

You’ll have to crop the image about 20 pixels, since to correct the problem, the GIMP displaces parts of the image and this will be visible in the corners.


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