The Dreaded 'CA's': Color artifacts and chromatic abberation

Another handy application of the color blending mode is to fix lens artifacts such as flare and chromatic aberration ('CA'). Lens flare and chromatic aberration often cause blue-yellow or green-magenta fringes at the edges of high-contrast subjects. Chromatic aberration and certain types of flare are caused by lens distortions that focus different wavelengths or different colors of light at slightly different locations. Accordingly, these artifacts often do not significantly change the overall exposure.

While many image editing applications such as Adobe Lightroom have tools to reduce or eliminate color fringes due to chromatic aberration, there are images that require more intensive intervention.

The image below was shot with a LensBaby and wide-angle adapter, a combination I find often produces very strong chromatic aberration.

This image is a very succinct demonstration of chromatic aberration. Detail of the image, showing strong color artifacts on many of the image edges.

In addition to the brushing technique show previously, we can use the clone stamp tool to help in these situations. Here, we will clone a nearby area with the proper color over the undesired color artifact. The trick here is to set the sampling mode to 'current and below', which will let you paint on your blend layer while sampling pixels from the visible image stack.

This is a very useful technique for painting on blend layers; selecting 'current and below' allows the clone stamp tool to paint on a new layer using the pixels from lower visible layers. The healing brush tool supports this option as well.

Cloning color from the model's arm allows us to quickly remove chromatic aberration. For other parts of the image, such as the eyebrow and eyelashes, it would be easier to clone the white background or simply paint white on our blend layer to remove the unwanted color artifacts. This would cause the color fringe to go to grey; try it for yourself on the large copy of the image.

To remove the CA, we first set the sample point for the clone stamp too to an area of 'good' color. We then paint over the color artifacts, being careful not to bleed onto the white background.

'Overlay' Blending Mode

Another often overlooked but very useful blending mode is 'overlay'. Intuitively, this blending mode will work much like sandwiching two negatives or photographic slides together.  Not surprisingly, it can be used to add textures or overlays to an image.

Popular with the kids

While the art process I used to create the darkroom image earlier in this article may not appeal to everybody, there's no mistaking the popularity of this type of image manipulation among some artists. Several of the products sold by some software manufacturers, for example, are little more than texture sets packaged with Photoshop actions to resize and overlay the textures on images in order to create exactly this type of effect. Let's walk through an example of doing this by hand.

This is a photograph I took of the masonry floor pan inside my shower at the charming Riad Miski in Marrakech.

Collections of textures similar to the image above can be found online by searching for keywords like 'photoshop textures'.

Using the above texture as a blend layer by setting its blending mode to 'overlay', we can instantly change the look and feel of the image. In many situations, this effect can look contrived, but it is nevertheless an effective complement to some images.

A sharp substitute

Photoshop's documentation for the 'overlay' mode is a bit difficult to understand. It reads: Overlay 'Multiplies or screens the colors, depending on the base color. Patterns or colors overlay the existing pixels while preserving the highlights and shadows of the base color. The base color is not replaced, but mixed with the blend color to reflect the lightness or darkness of the original color.'

Confused? This basically means that the overlay mode increases contrast; dark tones in the blend layer will produce darker resulting tones (multiply has an effect similar to drawing with multiple colored markers, one over the other), and light tones in the blend layer will produce lighter resulting tones (screen has the opposite effect of multiply).

An image sharpened using a non-destructive overlay sharpening layer.
A more subtle use of the overlay blending mode is for easy, non-destructive image sharpening.  The sharpening filters built into Photoshop have certainly improved over the years; it's possible to get very good effects using unsharp mask or smart sharpen. However, the settings on these filters can be confusing enough that it's not uncommon to hear photographers recommend 'one size fits all' settings like 'Use Smart Sharpen at 100%, radius 0.2 pixels, Lens blur' for everything.
 
How can overlay blending help us to do better? There is another unassuming filter in Photoshop called 'high-pass filter'. According to its documentation, this filter 'retains edge details in the specified radius where sharp color transitions occur and suppresses the rest of the image'.

Here are the steps to the overlay blend mode for sharpening:

  1. Copy the image into a blend layer. 'Merge stamp visible' (Ctrl + alt + shift + E / Command + option + shift E) works great for this!
  2. Use the high pass filter and adjust the radius until just the edges of the image are visible.
  3. Desaturate the blend layer (Ctrl + shift + U / Command + shift + U).
  4. Set the layer's blending mode to 'overlay'.

This effectively 'traces over' or darkens the edges in the image, which increases perceived sharpness.  In some circumstances, it can also perform better than unsharp mask, which sometimes enhances digital noise.

The 'high pass' filter is located under 'Filters' -> 'Other'. To sharpen an image, adjust the radius until just the edge detail is visible.

The sharpening layer be selectively added or removed with layer masks; the strength of the effect can also be changed globally by reducing the layer's opacity.

This is a detail view of the image prior to sharpening. Sharpening layer applied at 100%. Note that the effect also enhances pores in the skin; a layer mask can be used to limit the effect to the lips and eyes.

Surprisingly soft

Perhaps surprisingly, the same technique can also be used to soften images. The trick there is to invert the image in the blend layer (Ctrl + i / Command + i). Rather than 'tracing over' the edges in the image, the inverted image will cause the edges to be partially lightened, thereby softening the image. One common use of this technique is to reduce pores or improve the appearance of rough or blemished skin.

This is an inverted copy of the same high-pass blend layer used to sharpen the image. Here, the inverted layer has been applied to the image. Again, a layer mask could be used to target only specific areas of the skin.

Conclusion

Although this article is certainly not a comprehensive exploration of all of the blending modes available in Photoshop, I hope it shows off some powerful techniques for retouching and restoring your images, all of which are available without the need to purchase expensive plug-ins or filter packages.


Thomas Park is a fashion / fine art photographer and educator based in Seattle, Washington. To follow his work, please visit http://www.thomasparkphoto.com.

Cast and crew Models: Alison Douglas, Evangaline S., Ari DiMatteo, Karren Schafer, Erin A. Ari appears courtesy Seattle Models Guild. Hair and makeup by Phoebe Goodwin, Danyale @ Pure Alchemy SalonAmy Gillespie, Calvy Tran. Wardrobe and fashion styling by Jenn Burland, Wai-Ching, Thomas Park.