Recommendations to avoid color fringing

Question

I'd like to have suggestions to prevent color fringing as much as possible; ideally, when taking pictures, i.e., before post-processing.
I understand that overexposed images may cause color fringing, or even harsh light, from what I can understand.

It happened to me, though, to see it even when shooting in the shade (but the sky is harsh bright). I'm not saying I see it everywhere, but it can definitely appear.

PS: I'm shooting RAW, with no DSLR pre-sharpening (Portrait mode is Faithful), Canon T4i and L-lenses (this question mostly relates to Canon 50mm f1.2L).

Answer 1

There are three main types of colour fringing:

• Lateral chromatic aberration. This is the result of the lens focal length differing depending on the wavelength of incoming light. It is seen mainly in the corners and can be readily corrected, either by the camera (in JPEG mode) or by the RAW conversion software. Better lenses show less lateral CA but in the world of digital it's not the problem it once was.

• Longitudinal chromatic aberration otherwise known as axial colour. This is the result of different wavelengths coming into focus at different distances, resulting in out of focus highlights having a magenta tint in front of the plane of focus and a green tint behind. This is much harder to correct as the camera/RAW software doesn't know what is in front or behind the plane of focus. Better lenses exhibit less axial colour, and it disappears stopping down but fast lenses all show some degree of axial colour wide open.

• Purple fringing. This is the result of axial colour in the infra-red spectrum being picked up as the blue and to some extend red dyes used in the sensor CFA both pass IR resulting in a purple glow around highlights. It can be removed in software and in theory can be prevented by using an IR cut filter on the lens, though I have never tried this.

Unfortunately ultra-fast lenses like the EF 50mm f/1.2L are going to exhibit all of these aberrations at f/1.2. So if you're shooting for shallow depth of field you're going to have to remove them in software.

Answer 2

The purple/red/green thingo on fast primes that appears in any part of the image occurs only on bits that are out of focus. This happens cos the colours don't all reach the sensor at the same focus, so some colours are front focussed and some backfocusses. It seems to be more noticeable on bright borders.

You may either postprocess them away, or stop the lens down so they go away, or ... sadly, suck it up. Alternatively, don't use that lens on high contrast situations where the OOF colours will annoy, ie use the lens in dark environments.

On the digital picture tools thingo, you can see the colour fringing on fast primes with the resolution thingo. Look at the 100mm f2, the numbers on the chart clearly have a purple halo.

:(

Answer 3

Color fringing (chromatic aberration) is caused by the fact that different wavelengths of light do not refract (and therefore do not focus) the same. More expensive lenses with lens coatings and achromatic elements can reduce CA. Cheaper lenses, particularly zoom lenses, tend to display more fringing. With a given lens, you can reduce fringing by using the sweet spot of the lens (stopped down a few stops), where you'll have the best focus, rather than wide open or near the minimum aperture.

Many post-processing programs like Photoshop, can reduce fringing automatically, with or without a lens profile.

Answer 4

Fast primes, even expensive ones, will show CA at wider apertures under certain conditions. The best way to deal with it without giving up the wide aperture is in processing, either by the camera's jpeg engine or in post.

To use the lens profile from Canon you can use Digital Photo Professional to convert the RAW files after adding the profiles for your lenses. You do this via the updater button under the lens tab on the tool pallete in the edit image window. You must have an image selected to open the edit image window. If the tool pallete is not visible, click view->tool pallete. Select the lens tab and click the update button. Once the list appears you can select as many lenses as you wish. Not all of Canon's lenses are on the list, but most of the higher end EF-S lenses and current "L" glass with focal lengths below 100mm (except the Tilt Shifts and Macros), and the Super Telephotos other than the EF 800mm f/5.6L are there.

Other RAW converters such as DxO also have detailed lens profiles for many lenses.