Expose to the Right and Chromatic Aberration

[Jonathan Wienke]

Changing your exposure won't fix chromatic aberration. If you underexpose enough so that the bright object doesn't show fringing, the dark areas will be clipped to black. theimagingfactory's Debarrelizer plugin works great at fixing CA, and increases sharpness too.

[Jonathan Wienke]

That may depend on the lens. My 35-350mm has CA at the wide end, and it seems about the same at f/16 as  f/5.6 @ 35mm. Your mileage may vary, you would have to test your lens.

[Ray]

Is the direct cause of the CA and purple fringing large apertures or  high exposure level? I ask because the two factors often go together, and the wrong one might get blamed.

BJL,
It's unfortunate that stoppng down does not cure all ills. Chromatic aberration is generally considered to be one of those types of aberration that is not[/i] reduced by stopping down.

[dlashier]

> But for images with areas of high contrast, I need to move back to the left to avoid the aberration...

Yes, this can be a problem unrelated to lens CA. I think the problem is channel clipping where the raw convertor can't properly determine an edge color because an adjacent R or B is blown, or in the case of a high contrast edge the possibility also exists where one of the adjacent pixels has a 0 (what do you call blown on the low end?). ARC seems to have more of a problem with this than C1 although the latter exhibits it also.

- DL

[mhawkins]

The "Expose (to the) Right" article makes an excellent point about using the available dynamic range by exposing toward the high end.  But I find that in practice, many digital cameras (even digital SLRs with high end lenses) exhibit chromatic aberration (the dreaded "purple fringing") in high contrast areas of the image.  So if a tone in the top fifth of the exposure range appears next to a tone in the bottom fifth, there will be a line of purple (or some other color depending on the camera) in between.  This isn't limited to tones which are blown out, but seems to occur for any highlight near the top of the range.  This can have such a negative impact on the image quality that I often find myself reducing the exposure when shooting high contrast subjects in order to avoid it - the opposite of what I should be doing according to this article.  In images where the bright and dark tones have smooth transitions in between, this would not be an issue and I could "expose to the right".  But for images with areas of high contrast, I need to move back to the left to avoid the aberration...

[BJL]

Is the direct cause of the CA and purple fringing large apertures or  high exposure level? I ask because the two factors often go together, and the wrong one might get blamed.

If aperture size rather than the total exposure is the villain, the solution could be to avoid the big apertures (what's new?) and adjust exposure level mostly with shutter speed and ISO setting.

[mhawkins]

Thanks for the response, I'll give Debarralizer a try.  I've also noticed a very significant difference among RAW converters in their ability to remove CA.  Surprisingly, I've found that Canon's own RAW conversion tools are the best at removing it, even compared to otherwise superior tools like Adobe Camera RAW and Capure One DSLR (and yes, I've double checked that I have their "moire filter" options enabled when testing them).  I like being able to minimize the CA right in the RAW conversion process, since it avoids the extra step of using some other software to remove it, so as painful as it is I've actually gone back to using the Canon RAW conversion utilities.  It'd be great if some of these other RAW conversion programs could improve their CA removal algorithms.  But since it looks like Debarralizer is a Photoshop plugin, the combination of Adobe Camera RAW and Debarralizer sounds like a good solution, since I can do everything from within Photoshop.

[BJL]

Thanks: so it seems from the comments of those who probably know that CA does not decrease with smaller apertures, at least in SLR's; but on the other hand, Phil Askey's reviews of compact digital cameras at DPReview often indicate that CA decreases on them with smaller aperture. Could that be because with the rather small focal lengths and tiny pixels of such cameras, significant diffraction effects set in at their smaller aperture ratios and blur the image over several pixels, smearing out any CA?

P. S. At least some of that digicam purple fringing seems to be a sensor issue rather than just optical CA: it changes significantly on new models with the same lens. So perhpas i have been confusing two different types of "color" abberation.