How to minimize/avoid purple fringing before/during shooting?

Question

I know it is possible to avoid purple fringing using post-processing software's chromatic aberration correction capabilities but I have a few pictures where the one I'm using (Rawtherapee on Linux) cannot fully remove it.

Most of the pictures where I experienced a lot of PF are taken against the light from sunset reflecting into water. Generally it is not visible on the JPEG preview of the camera so I suppose it may be caused by rawtherapee not being as efficient as camera and/or Adobe Lightroom post-processing. However for a few pictures it is so strong that even the camera JPEG preview experience a lot of PF.

For example this picture where all lights should be white is the JPEG extracted from the raw file produced by camera with a Nikkor 50mm 1.8 and I have the exact same result using a Nikkor DX 35mm 1.8:

So my question is how to avoid those PF, or at least how to minimize them before post-processing (i.e. before or when shooting). When shooting against sunlight can a polarizing filter help? I also read that some UV filters seems to be able to reduce them. What about them? And also what could I do for something like the above picture taken at night?

Answer 1

The purple tinge to the lights in your photograph has been caused by longitudinal chromatic aberration, which results when using a wide aperture, meaning the blue and red components of the spectrum are not focused as sharply as the rest. It can also manifest as a green tinge, or even both where there are specular highlights either side of the plane of focus.

To reduce longitudinal CA, use a narrower aperture, or if that's not an option, try to focus manually, using live view if your camera supports it. The fact that all of the lights on the left hand building are showing the same amount of purple fringing suggests that this might be fixable with more accurate focusing (though this might result in green tinges to the lights on the building to the right).

More generally, lateral chromatic aberration, which can result in purple fringing at the edges of the frame, is as Mitch says in his answer, an attribute of the lens and cannot be fixed in-camera (notwithstanding lens profile corrections which apply CA reduction via camera software).

Answer 2

Purple fringe most commonly happens with fast lenses used wide open shooting directly into a source of light. They generally show up around areas with blown highlights, so the two easiest things you can do to reduce fringing are to a) stop the lens down to a smaller aperture, and b) avoid blowing any highlights.

Purple fringe is also known as longitudinal chromatic aberration (CA) or "bokeh CA", and is caused by the different frequencies of light passing through the glass differently and focusing at different points. Longitudinal is front-to-back displacement, "lateral" (most commonly cyan/red, and is what is most often referred to as CA by correction software, vs. "fringing") is side-to-side displacement. With LoCA, the focus point can affect the CA. It looks as if the building on the left is slightly out of focus, but if you had adjusted and focused on it more accurately so its lights were white, then the building on the right--depending on where it's placed in the depth of field, might have then shown the purple fringing or green fringing.

Answer 3

Chromatic aberrations are caused by different wave lengths (in this case representative of colors) being manipulated slightly differently by your lens and thus arriving at the sensor in slightly different locations.

As such, they are intrinsically attributes of your lens. You can minimize them by not adding unnecessary glass (cheap filters mostly) and by knowing your lens well. Your lenses will likely perform differently at different aperture settings and focal lengths. Knowing which ones have minimal chromatic aberration can allow you to optimize the exposure when aberrations may be a problem.

Otherwise, you could avoid high contrast situations that exacerbate chromatic aberration, take care of it in post processing or look into a new lens.

For the posted photo, you could likely manipulate the colors directly to fix it, rather than trying to do it via CA removal. This wouldn't be an option in a photo with more light, but since all the lights should look roughly the same it would be a relatively easy adjustment.