Chromatic aberrations (commonly called colour fringing) are one of the most common and yet annoying photographic artifacts that occur in digital images. You’ll most likely find them in high contrast regions of your photos such as the edges of buildings or the branches of trees. They are caused by the fact that the visible spectrum is made up of not one but six monochromatic colours, (green, yellow, violet, blue, orange red) each with a unique wavelength. These differing wavelength alter the speed at which each colour enters a cameras lens, since the refractive index of the lens is wavelength dependant, and so dispersion of the colours can occur. This dispersion can be minimised by using specialist equipment such as an achromatic lens but that’s not much good advice if you’ve already shot a couple hundred pictures and now find green and purple fringing present. Don’t worry, there’s always hope! In Photoshop CS3 there are a number of ways to remove colour fringing with just a little effort.
1. The Sponge Tool.
The Sponge Tool in Photoshop can be used to either saturate or desaturate colour and is used in exactly the same way as the eraser, so is ideal if you want to remove small regions of colour fringing. It can be found in the tool bar as shown here on the right. Depending on Photoshop’s default settings the dodge tool or burn tool might be visible instead. Once you select the sponge tool you can set the brush size, choose saturate or desaturate, and choose the flow percentage (the pressure that the sponge is being pressed against the picture.) These settings should be visible just above the image, although it might be different on a windows machine – I am using a Mac. In any case, once you are satisfied with the setting you can sponge over the affected regions (using desaturate I would imagine) and gradually fade the fringing away to a non-noticeable greyish colour. It’s probably best to begin with a fairly low flow percentage (maybe 40%) and increase it once you gauge its effect.
2. Adjustment Layers.
This is most effective if you want to remove a lot of fringing of a particular colour in one swoop. When you first open you image, if the layers palette is open (Window -> Layers), you image should be present and called Background. It will also have a lock symbol beside it. Double click the image to convert it to a layer. Then add a Hue / Saturation layer (either by Layer -> New Adjustment Layer -> Hue/Saturation or adding it from the options at the bottom of the layers palette). The box below should appear.
What we want to do now is, using the Saturation (and possibly the Brightness) scale, desaturate all of a particular color fringing that is causing us grief, e.g. purple. “Master” refers to the entire image so lowering the saturation will desaturate the everything – not what we want! Click edit and change it any other colour, it doesn’t really matter which one since we’ll be choosing the colour we want directly from the image using the eyedropper – make sure the first one is selected in the Hue/Saturation box (see above) and select the colour we want to desaturate. You might have to zoom in first. Alter the brightness also if the desaturation is too bright.
Once this step is complete a mask layer might be necessary if any desirable purples in the image were unintentionally desaturated. In the Hue/Saturation layer to the right we can see a second blank box. This is the mask layer and will negate the effect of the adjustment layer that it’s attached to – it masks it, geddit? There’s more to masks but for the moment all we have to do is select the paintbrush, choose black as the paint colour and brush over any region in our image that we don’t want to be affected by the adjustment layer (it’s a good idea to turn on and off the adjustment layer using the eye symbol beside the layer to see what parts of the image were affected and which parts we have to mask).
Two easy ways to remove colour fringing!
