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Colour fringing with Patricia Fenn - If you like vibrant colours in your work then learning about colour fringing is something you should definitely do.
Chromatic aberration, or colour fringing, is a perfect if your style is to boost the colours in an image to just short of the ridiculous, as I do.
The fringing is caused by a lens having a different refractive index for different wavelengths of light .
Longitude and latitude chromatic aberration in a lens is seen as "fringes" of colour around the image, especially around the perimeter and it's even stronger in the corners. This is because each colour in the optical spectrum cannot be focused at a single common point on the optical axis.
High contrasts, long lenses and high f stops exacerbate the problem and any saturation of the image also saturates the fringing.
There are several editing programs that claim to deal with the problem but so far I haven't found one that gives a really satisfactory result so I have developed my own way of dealing with the problem.
In a highly saturated image with strong contrasts the fringing will be quite prominent. In the image to the right the magenta/red and cyan can be seen along the ridges of the pillars.
This principle can be applied to most fringing problems:
1. Open the edited image , give it a name and save as a tiff file.
Make a duplicate layer.
2. Zoom in to the top layer, 100%, and with the eraser set at a diameter of 12 pixels and a hardness of 3% erase a strip of fringing.
3. In the layers pallet click on the bottom layer.
4. In the menu click Image>Adjustment>Hue/saturation. Click Colorise. You will see thru the erased hole that the bottom layer has changed colour.
5. Now move the Hue slider until you get a good match with the top layer and adjust the saturation and lightness sliders until the best match is achieved. In this case I used Hue 36, saturation 37, lightness 5. Click OK.
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6. Now, in the layers pallet, click on the top layer and with the eraser set at between 30% and 75% erase all the fringing in that colour range.
7. Merge down and check for more fringing in a different colour range, for example on the edges of trees. Use the same process once again matching the colorised bottom layer to the green of the trees in the top layer.
8. Merge down and save.
Words by Patricia Fenn.