All images are (c) 2004 by Tels. Photos taken with a Panasonic DMC-FZ2 and post-proccessed with The Gimp.
Feedback of any kind is very welcome, although flames will be handled by Mr. Dave Null. Please use GnuPG with my key when emailing me.
CA or Cromatic Aberration (see here for an introduction) is an often seen, but usually unwanted effect in (analog or digital) photography. Especially when using long lenses and/or teleconverters the CA can become a problem.
There exist different techniques to get rid of it. I present here one I found by accident and which works very well for certain types of images.
Below you'll see the unaltered image, straight from my camera, except for resizing it to 800x600. Note the bad CA in the upper left corner and around all the dark branches against the light sky. This is a typical problem image.
The technique is quite simple:
The only complicated step is getting the right selection, and even this
can be trained. The rest is easy.
Some people prefer to select only the cyan or green parts and remove these
colors. Unfortunately, this doesn't work with the Gimp (there seems to be
no way to select "by color" across the image) and it also does not produce
so good results than my technique, IMHO :)
If you use a different program than Gimp, you will adjust some of the details when replicating the technique, but it should work almost the same.
Here are some details on working with the Gimp:
The following shot shows how I selected some areas with the Fuzzy
Selection tool (sometimes called the Magic Wand).
I colored the area light red so that you can better see it. I am not sure if
a newer Gimp can do this automatically, this would be great for seeing
your selection. In the meanwhile you have to color it, then reverse this by
pressing CTRL+R.
Note that the Fuzzy Selection selects pixels that have the same or nearly the same color, so you might adjust it's properties a bit. Just clicking on areas you want to select multiple times to get more selected.
In addition ou can press shift (this will add newly made selections to your current selection) and use the rectangle selector. Here is a shot after I selected most of the problematic area (and colored it light red to show it better).
One further trick is to grow the selection by one pixel, this will
encompass and thus get rid of tiny parts that are not selected.
On some images, like the one we have here, it is actually easier to select
the parts you don't want to change and then simple invert the
selection.
The goal is to come up with a selection (or mask) that covers all the pixels you want to change, and does not cover any of the pixels you don't want to change.
After fully selecting the area to work on, we desaturate it:
Because a perfect gray is usually not what the original was, we need to recolor the image. Just keep the selection and use whatever method you prefer.
I used the Color Balance tool and adjusted the highlight and shadow tones for red and green until the colors matched the original color tone very closely. Save your work and you are done.
Below you'll see the unaltered image, straight from my camera, except for resizing it to 800x600, and the final result. Click on the images to see larger versions.
Here are some crops of the upper left corner:
Before |
& |
After |
Feedback of any kind is very welcome, although flames will be handled by Mr. Dave Null. Please use GnuPG with my key when emailing me.
I would like to thank the kind people on the DPreview forums, especially the many people posting their pictures and experiences.
Back to my hardware page.
|
Tels Created: 2004-01-25 Last modified: 2004-01-25 |
|