Chromatic Aberration?

[photog1107]

What the heck is chromatic aberration and how do I avoid it? I just did my first shoot in RAW and a LOT of my images came into Lightroom with the colored "fringe" around all the image areas. The only thing I could find to fix it was Lightroom's Chromatic Aberration toggle, which helped once I pegged out the slider all the way to the right.

Did I do something wrong, or is this inherent in RAW? I can post a shot if you like, but I assume most experienced photogs know what I am describing... Thanks for any tips here!!

[kencaleno]

Chromatic aberration(color fringing) is directly due to lens quality-especially at the long end of kit zooms-best way to cope with this phenomenon,is to not extend zoom to full capacity. Red,Green and Blue (primary colors) have differing wavelengths,and to avoid fringing we need to have all 3 colors converging on the one focal point.Proper way is to invest in very expensive glass, called "apochromatic lenses", where doublet correction elements are employed to counterract Chromatic Aberration. here's how ,(two ways) I control it in photoshop:

- Lateral chromatic aberration (fringing) Removal

Ctrl+J select brush tool and click on the quick mask tool.
Choose a brush size to match the width of the fringe.
Draw all the areas that have fringing. When done, click icon next to quick mask
Select> Inverse.
Image> Adjustments> Hue/Saturation and pick the color closest to your fringe color Click eyedropper on the fringe color on your image.
drag Saturation slider to the left until the fringing goes.
Select> deselect
Save

Quick method for fringe elimination

Using lasso tool select area with fringing
Image> adjustments> Hue/Saturation
De saturate fringe color
Select> Deselect
save

[OsmosisStudios]

Chromatic abberations are a deviance of the light going through the lens based on it's spectrum wavelength.

Confused yet?

Light travels at different wavelengths (Ultraviolet to Infrared). Each wave has its own speed and properties. Sometimes, lenses make it so that certain wavelengths deviate from the line their supposed to follow: this results in "fringing", which is a coloured halo around edges, particularly where dark and light areas meet.

Many cameras correct for this in-camera (all the newest Nikons, for instance), but for some it has to be done in post, which is where RAW comes in real handy. Try getting rid of CA on a jpg and you'll understand why.

What it boils down to is that it's inherent in a lens: generally it affects wide-angle more than telephoto. I know the NIkon 10.5 fisheye is VERY bad with CA in the edges and corners and it frustrated me no end. But it's otherwise a GREAT lens. It's hard to avoid it: you can buy your lenses based on their CA characteristics, but that's boring as all hell and won't leave you with much of any interest. Just learn to live it, correct it, and enjoy shooting.

[photog1107]

Thank you so much for the education. I was shooting with my Canon 75-300 IS a great deal of the time on that shoot, and I am not sure why I have never noticed it to this extent before, so I thought it had something to do with the RAW. Would RAW make it show up more distinctly than a jpg? Sounds you you both are saying it's strictly due to the physics of the lens function itself.

At any rate, I do appreciate all the technical input and advice. Thanks!

[inkista]

Lateral CA, there's not much you can do about it--it's inherent in the lens, and you just correct for it in post-processing.

However, there's a second kind of CA which is most commonly referred to as purple fringing, but can also show up as red sensor bloom. This type of CA tends to show up on the edges of blown highlights. The explanation I remember is that it's a matter of a specific pixel "overloading", and the charge creeping from it to surrounding pixels. This is a combination of lens and sensor, and most commonly affects very fast lenses when used at large apertures (f/1.8-f/1.0).

The easiest ways to reduce this effect are to 1) make sure you don't blow any highlights (avoid shooting backlit things), and 2) to stop the lens down. In post-processing, you can desaturate purple to reduce purple fringing.

Stopping a lens down often improves its vignetting, CA (both lateral and fringing), and sharpness. Shooting wide open is nice for the bokeh, but it does have its cost with most lenses.