What it is & why it happens...
Chromatic aberration is an optical effect found in almost all wide angle and cheap telephoto lenses, both primes and zooms. It's caused when the different wavelengths of white light originating from the same point hit the sensor (or film plane) at two different points. It always appears as two different colours as a result.
Blooming cannot occur with film capture. It is caused by the micro lenses on a digital camera sensor and is compounded by individual photodiodes being overloaded with charge (blowing out to 255,255,255 white) next to low charge areas (dark tones). It's always a single colour, usually purple, sometimes blue'ish or red'ish. With digital captures it's not uncommon to have both chromatic aberration and blooming/fringing on the same image.
From here on in I'll refer to the lens issue as 'CA' and the digital sensor issue as 'blooming'.
Fixing CA
Above is an interiors image opened in Adobe Camera Raw (ACR). The red circle shows the area below under the 'Lens' tab. If you don't use ACR then in Photoshop go to: Filter > Distort > Lens Correction. This is a 200% view...
This is 'classic' CA. My 24mm lens always displays this red-green CA towards the horizontal edges and corners of the frame. With
ACR I just select all of the raw files that were taken with the 24mm in Adobe Bridge and paste an
ACR setting of -24 on the 'Fix Red/Cyan Fringe' slider to them all. Again opened in
ACR this is the result...
You can save settings and then apply them for your problem lenses/focal lengths, or use the copy & paste method (after correcting one image), or use the synchronize button within ACR. Whichever method you choose you can correct hundreds of images for CA in one fell swoop by selecting all of the raw thumbs in Bridge/ACR and then making a couple of mouse clicks. Neat eh!
Sometimes the problem is a little trickier to solve. My 17mm lens is a case in point. This is because it exhibits CA on more than two wavelengths of light and the CA effect shifts a little at different focusing points (between close focus and mid-infinity focus). Here's an image taken with the 17mm opened in ACR...
At 200% green-magenta CA is clearly visible...
This is corrected with a -17 on the 'Fix Red/Cyan Fringe' slider, and a +18 on the 'Fix Blue/Yellow Fringe' slider...

Fixing Blooming
Below is another 200% section of the above image with the Chromatic Aberration sliders still at -17/+18...
If we try to use the sliders to correct this problem we'll cause other problems elsewhere. The sliders are for CA only - they always manipulate two colours. This problem is blooming and it needs a different technique to fix it. Here it is at 200% in PS...

There's no raw conversions fix for this issue but it is easy to deal with in PS. Unfortunately it's a manual job. There are non-manual PS fixes but they corrupt the whole image unless you resort to manual control to limit their effect.
We're going to use the clone and/or brush tool to paint away the blooming. Create a transparent layer above the image layer (click the circled icon) and set this new layer to 'Color' mode...
Now set your clone tool or brush tool to color mode. This is the clone tool set up for working on the image...
Ensure that the 'Sample All Layers' box is ticked if you're using the clone tool. Sometimes it's best to use 'Aligned' cloning and sometimes it's not. It just depends on the amount of space you have to sample from in relation to the blooming.
If you're using the Brush tool (again a decision based on sampling area) ensure that you've sampled the foreground colour with the eyedropper tool from an area unaffected by blooming.
Now just paint/dab away. With either the clone tool or a brush you're simply overlaying the colour component of the sampled area/colour. The underlying luminosity (light, shadow & texture) is not affected. This is the job in progress with the clone tool (diameter 9px, hardness 70%)...
For the longer diagonal strip (not blooming, I just didn't like the colour!) I used the brush as the sampling area for cloning was very narrow. The whole thing took less than 30 seconds...

PS - In the 'Fixing CA' section the slider adjustments match the lens focal lengths. This is pure coincidence. Those sliders have nothing to do with focal lengths.
Labels: Post Production, Technique