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Chromatic aberrations. The name sounds scary, and while they’re not a life threatening issue they’re something you should really keep under consideration when it comes to photography, HDR in particular (more on this later).

The technical reasons behind the presence of CA in a picture can be a bit complex, but using Layman’s terms it’s simply the inability of the lens to reproduce the correct colors between two different objects/surfaces. The most common place to look for CA is the branches of a tree that has the sky as a background. Here’s an example on a picture that I’ve already used in my blog:

CAexample

Look at the areas between the branches of the tree, the leaves and the sky: it’s purple! Before you blame your camera and head to the store to complain about it, keep in mind a couple of things:

a) CAs are the lens’ fault, not the camera. The lens cannot read correctly the information of 2 areas in very strong contrast (dark green leaves/white cloudy sky in this case) and the sensor of your camera cannot do much about it.

b) CAs are a “feature” of the design of a lens. Some lenses are very prone to them, some are not and asking a store for a replacement will do no good. My 50mm f1.8 has very little CA, while the Tokina 11-16 f2.8 shows CA in almost every picture I take.

So, what can we do about it? There’s a few ways around it.

The easiest solution would be:”DON’T SHOOT SUBJECTS IN STRONG CONTRAST” but that’s just not possible because very often contrast is a key element in a picture. We need to rely on software.

Here’s the same picture as above processed in DxO Optics 9 Pro:

CAexample_DxO

Even if you zoom in at 400% and check the picture pixel by pixel you’ll notice that the purple fringing is nowhere to be found. How long did it take? Roughly 5 seconds. All you need to do is load your RAW file in DxO (Capture One and Lightroom will work fine too, but I personally prefer DxO), make sure the CA correction feature is turned on and you’re done. Since we’re in 2014 you don’t need to do this for every single picture, it’s possible to do batch processing which makes the whole thing much quicker.

But….what happens with HDR?

HDR pictures are the result of 3 or more shots merged together. In order to do so you’d usually bypass programs like DxO and simply use Photomatix to create your starting TIF file that you can further process in Paintshop/Photoshop. The problem is that Photomatix, and most HDR software in general, is not built specifically to handle things such as CA. It has some built in CA correction but it’s very limited and cannot keep up with heavy CA extended on 5 pictures, and this is the result:

CAhdr

Oh snap, the sky is a mess.

In order to handle this you need to add a step in your processing workflow right at the beginning: instead of loading your 3 (or more) exposures straight into Photomatix, load them in DxO and convert them as TIF files (not JPG, you’ll waste a lot of information and the final result will look terrible). During this process make sure you don’t let DxO adjust the exposure or it will be impossible to create an HDR. DxO by default wants to give the right exposure to your pics, it doesn’t know that you took a -2 shot on purpose.

Adding this step in your workflow means that Photomatix will work with 5 CA-free TIF files, rather than 5 RAW files full of CAs. And this is the final result:

Stairway-final-jpg

As you can see the final picture is completely CA-free.

Almost every editing/conversion software has some CA removal feature, but not all software will work with the RAW files straight out of your camera. If you shoot JPG (don’t do it, please) and try to remove CA in Photoshop, Paintshop or whatever, the final result will not be even comparable to what you can achieve by working on RAW/TIF files, and this applies not just to CA but to any kind of editing or correction that you want to apply to a picture.

So, grab your camera, get outside, find a tree and take a picture of it using the sky as background. Load your RAW file on the pc and check: is there any purple fringing? If the answer is yes: congratulations! Welcome to the wonderful world of chromatic aberrations!

Note 1: sometimes CA can show up in different colors, not just purple

Note 2: as I often mention, WordPress does not allow to upload RAW or TIF files, so what you’ve seen here are all JPG files.

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