It's been a while since I posted in the 101 series. I'm trying to get back into it but I've just been so busy. Still, I've got a pile of these things floating around in my head - help! I need to get them out! :)

Anyway, I wanted to talk today about a gremlin of sorts that impacts anyone with a camera, period. I'm talking about Chromatic Aberration also known more informally as Color Fringing.

CA most frequently occurs when light entering a lens is of high contrast, i.e bright light next to low light or no light. At the exact edge of the contrast you'll often see super saturated 'mystery colors', usually cyan or red...but sometimes purple, blue, green, or even yellow. Yup, that's CA for you.

Let's take a look:

To the left is a 100% crop of a tree against what is  essentially a white sky (more lovely Seattle weather).

Yikes!

Look at all the CA. You can see that we have  quite a bit of cyan around the right edges of almost anything that's high contrast...and we have red on the left side.

BTW, trees against a bright sky is one of the number one worst CA culprits. It's very difficult if not impossible to avoid in this kind of scenario. It's also very common aroind windows and buildings (hint).

So what do we do about it?

We actually don't have too many options. Here's a list:

1. Never photograph any high contrast light: Sort of like abstinence is the only way to really be sure you won't get pregnant...and about as popular. Still CA is going to be with us more or less always (at least at our current technological level) so you might as well get used to it being present to some degree.
2. Buy a perfect lens: If only it was possible. Unfortunately there are no perfect lenses. But it brings up an important point. CA primarily occurs because light entering the lens gets split by the various elements and as a result doesn't hit the sensor in a uniform / cohesive way. Imagine 6 cars driving down a six lane highway...that cyan colored CA car is veering off slightly on the shoulder. That said, expensive professional grade lenses resist CA much better than consumer lenses in most cases.
3. Stop down your aperture: All lenses have a 'sweet spot' where they perform best. You might need to experiment a little to discover where this is. Generally it's going to be in the middle apertures of the lens (example f8-f16 etc). This will be the area where the lens will usually be it's sharpest, and most distortion free (including reduced CA).
4. Photoshop Method 1: Photoshop actually includes a function that helps reduce CA. It works...ok. Which is to say it can be helpful but it's not solution by any means. You can find it both in the raw converter as well as in the lens correction filter. Basically it's a little slider that allows you to apply the opposite color to certain color wavelengths...just along the edges of things. It's useful but don't push it too far or you'll simply be exchanging one CA color for another...also watch the CA on the other side as you can actually make it worse!
5. Photoshop Method 2: If the CA is happening in an area where it's not against a similar background color you can simply select that area, use a hue and saturation (or curves) and reduce or eliminate the saturation of the offending color. This is a little more time consuming, won't always be an option depending on background color, but it can be very effective.  

I should mention one other strategy...just ignore it. :)

I'm a perfectionist of sort...but most of the time (especially when the images are small, say MLS size) people won't notice CA...or if they do it won't really register as anything significant.

I like to have the cleanest image possible of course, and you may find yourself in a similar boat when that perfect shots is exhibiting these annoying little edge colors vis a vis a 70's acid trip. Or not. In any event, now you know what it is and what can be done to address it.

:)

Cheers, -B