After taking this shot I was quite looking forward to seeing how it came out. I was using my Hoya Circular Polariser at the time and I’d rotated it to bring out the best of the colour both in the sky and on the rocks – the green of the seaweed was really emphasised by it and it was one of those rare moments when I was really glad I’d bought it.
Sadly, when I viewed the shot on the computer, this is what it looked like…
Notice the sad face being pulled by the horizon – I’d forgotten that the Sigma 18-200 has really noticeable barrel distortion (where straight lines are bent away from the centre of the image) at the wide (18mm) end. That’s not to say that it’s a bad lens – for the price it’s a pretty good all rounder.
Fortunately it’s possibly to remedy this (with a bit of trial and error) with The GIMP.
Versions prior to 2.4
Open the file and select “Distorts” from the “Filters” menu. Select “Curve Bend” from the sub menu that appears. You should then see something similar to the following:-
To correct the barrel distortion we need to build a curve which will be used to modify the image. To do this I first placed a straight line roughly where the horizon was in my shot by clicking on the left and right sides of the grid. Then I placed a point (by clicking in the desired place on the grid) in the centre of this line which I then I slowly dragged downwards, changing the straight line into a slight curve. Once the results of the preview looked acceptable I pressed the “Ok” button and waited for The GIMP apply the changes (which took about 15 seconds).
Versions 2.4 and up
A new filter was added in version 2.4 – Lens Distortion. Open the file and select “Distorts” from the “Filters” menu. Select “Lens distortion” from the sub menu that appears. You should then see something similar to the following:-
As the distortion on my original image was convex (bulging outwards) I needed to apply some concave distortion. To do this I tweaked the “Main” value and eventually settled on a value of -12.
From the manual…
Main – The amount of spherical correction to introduce. Positive values make the image convex while negative ones make it concave. The whole effect is similar to wrapping the image inside or outside a sphere.
More documentation on this filter can be found here – http://www.mirrorservice.org/sites/docs.gimp.org/en/plug-in-lens-distortion.html
