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In this movie we are going to look at a filter that will help us reduce chromatic aberration where the blue and red color fringing we sometime see on the edge of eye contrast parts of images. Now this is an issue that occurs with lesser quality lenses in microscopes and the creation of these color fringes are due to the different wavelength of light focusing at slightly different points. Photoshop has a filter that allows for some reduction of the artifact. It may or may not fix the effect completely, but can definitely improve the appearance of the image. If you are working along with me, open the image Kohlerred.tif found in your Exercise 8 file folder.
Here we have a grid pattern and the first thing we are going to do is correct for exposure issues. We are going to go to Image > Adjustment > Curves and here we can see that only a narrow part of the entire dynamic range is used. And in this case I would like to see a high contrast image for my calibration standards. So I will set a black point and you might also want to check the area size you are going to sample. Hold down the Right key or Ctrl key and we want to reduce this to a 5x5 average, so we are truly just selecting the area that is black.
We can move it over one of the black grid lines and now we are going to set a white point. We will use the set white point eyedropper. Select that. Here we are going to move to an area that we know should be white and we can see that maybe we could set that a little better. So we click little closer and that gives us a nice high contrast image and we can start to see the chromatic aberration. We can see the blue and we can see the red and this we will be fixing using a filter built into Photoshop.
So we are going to hit OK. Now I always have trouble finding this filter. It's not in a logical place. You have to filter and then distort and instead of causing distortion you are actually trying to remove distortion. So that's how to remember it's under Distort, under the Filter menu item and it's the Lens Correction filter. Now the Lens Correction filter allows you to not only adjust for this chromatic aberration, but also if you have distortion you can modify that. So in this case grid line should be lined up perfectly. Just to briefly cover that, all you do is move over to the Remove Distortion and you are able to manipulate the curvature of the image so that it matches gird lines.
More importantly is Chromatic Aberration controls. The first thing we will do is zoom in so we can see the color fringing a little better and we will look at this square here and we move this to the middle so we can center it and we can see the red and the blue. Now by using the Fix Red we are going to try to reduce the amount of red fringe. We will move this to the right and that is only making it more intense, so instead we will be reducing the red by moving it to the left.
Here you can see that there is a bit of a balance that when we remove the red we seem to accentuate some cyan. Now we are going to see about how that we can reduce the blue. We are going to bring that down and it's modifying these two sliders. We are not going to completely fix this problem, but by using Chromatic Aberration adjustments we are able to make the red and the blue fringes less severe.
Once you do this you are able to save it as a preset and apply it to an experimental image. You would use this pull down menu, Save Settings and then give it a name, in this case 40xgrid and hit Save. Now you will be able to load it by going to the same menu, Load Settings. And here it is. Lens Correction-40xgrid. I will cancel that. We are going to apply this by hitting OK.
Now I can see the color fringing while we're there is reduced quite a bit. So if you have a problem with color fringing, remember under Filter > Distort is the Lens Correction filter that you are able to correct for some color fringing as well as if there is spherical distortion, you can also correct for that.
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