Lightroom CA settings thread

[youpii]

Some of you are sharing some lens CA settings for Lightroom in different threads.
What about centralizing all that information into a "sticky" thread?

[David Kilpatrick]

Since it is not possible to store lens settings (except by creating an overall setting or a camera default) in ACR or Lightroom, it may be worth sharing my method:

First, the Red/Cyan slider is nearly always the only required correction; when Yellow/Blue is needed, it is often the reciprocal of Red/Cyan, or a lower value - never, in my experience, on its own or at a higher value. I don't know why the slider controls Red/Cyan as the actual fringes look Magenta/Green to me, but this is the right one to use.

Secondly, using ACR settings as the basis (Photoshop 'Lens Distortion' uses OPPOSITE plus-minus value so beware!), there are some common values for non-apochromatic zooms which seem to be a good start point:

For any ultra-wide angle zoom (10mm, 11mm, 17mm on full frame etc) use minus RC values around -25 to -30, lowering to around 15-20 at the long end of the zoom. The same applies to the 16mm end of the 16-80mm and 16-105mm in the range 16-35mm. Lenses with strong CA may need up to -50, lenses with minimal CA like the Sigma 12-24mm may need only -10 or so even at 12mm - but most wide zooms seem to hover around the -30 mark.
For any wide-to-tele zoom, using minus RG values around 15-20 at the short end. Again, some like the 16-XX zooms may need a stronger wide setting, others like 35-XX zooms a weaker one at the wide end.
For the range 35mm to 60mm including macros and primes and the mid range of the 16-80m 18-250, 28-75, 28-105 etc, zero settings will usually be needed.
For 70mm upwards, use plus RG settings. A convenient start point is to take 1/10th of the focal length as the setting - for 100mm, use +10RC - for 200mm use +20, 300mm +30 etc.

Always select defringe highlights, and set Chroma NR to around 50. If you are using Fill Light or Highlight Recovery, do not select defringe all edges, as you will get a type of contour round some edges which is artificial. Examine results at 100 per cent at the end (not corner) of the image where the CA is sharpest defined.

David

[Winston]

You can create named presets in Lightroom.

For example, if you have some settings that you consistently use for your Sony SAL1118, just save a preset with those settings named SAL1118.

If you use different setting for different focal lengths as David suggest above, then create several presets with names like SAL1118-11, SAL1118-15, SAL1118-18, etc.

[Dr. Harout]

Probably a not so intelligent question: does the aperture change act on CA on a specific focal length?

[David Kilpatrick]

Changing the aperture generally softens or hardens 'good lens' CA, but does not always affect its extent. The same correction will be used for example on the 16-80mm CZ, which has CA but it is very sharply defined, at different apertures. With cheap lenses, CA may reduce on stopping down. It's very difficult to judge the right correction for CA with a soft image (wide open) as the CA is lost in the general softness, it's best to stop the lens down to a medium aperture (say f11) and then you can see the fringe clearly, and see it moving when you adjust the correction. My suggested settings are based on apertures around this value.

David

[01af]

I don't know why the slider controls Red/Cyan as the actual fringes look Magenta/Green to me, but this is the right one to use.

It's because the CA correction works by expanding or shrinking the red channel (Red/Cyan) and the blue channel (Yellow/Blue) of the RGB image by a few pixels. The green channel will always be left untouched. So the sliders might as well be named just 'Red' and 'Blue'---mentioning Cyan and Yellow in the sliders' names is just a reminder of what the complementary colours of red and blue are. Pushing the Red/Cyan slider in one direction basically is just the same as shifting an (imaginary) Green/Magenta slider to the opposite direction.

Probably a not so intelligent question: does the aperture change act on CA on a specific focal length?

There are basically two kinds of chromatic aberrations; lateral and longitudinal.

Longitudinal chromatic aberrations (LoCA) turn fine detail before the plane of focus into one colour (e. g. magenta) and into the complementary colour (e. g. green) behind the plane of focus. Stopping down the lens will remove or reduce LoCA. LoCA cannot be affected by Lens Corrections at post-processing time.

Lateral chromatic aberrations (LaCA) add fringes of one colour (e. g. magenta) to bright-to-dark edges and add fringes of the complementary colour (e. g. green) to dark-to-bright edges. The farther away the edges are from the image's center, the wider the colour fringes will be. In digital imaging, LaCA can also be caused by the sensor's micro lenses which will add to the LaCA caused by the lens. Micro-lens LaCA depends on the angle of incidence of the rays of light, so it'll be stronger with wide-angle lenses and short-distance exit pupils. Some sensors are more prone to this than others. Stopping down the lens won't reduce LaCA---to the contrary, the improved contrast often will even emphasize the colour fringes. LaCA can be removed or reduced by Lens Corrections at post-processing time.

Please note that blooming or purple fringing in digital imaging, which is caused by photons spilling from over-exposed highlight edges to neighbour sensels, will also create nasty colour fringes of one single colour (usually blue, violet, or purple; hence the name), not a pair of complementary colours. These fringes must not be confused with those caused by LaCA! The width of the purple fringes depend on the degree of over-exposure and not on the distance from the image's center. Purple fringes caused by blooming cannot be removed by Lens Corrections at post-processing time but Camera Raw's/Lightroom's Defringe feature will care for them, at least to a degree. You can also avoid them at image capture time by reducing exposure.

In any given image, all three kinds of chromatic defects, or any combination thereof, can occur at the same time. So attacking them in post processing isn't always an easy and straightforward task. You must handle each kind separately. In particular, you must not try to remove purple fringing via the CA sliders.

-- Olaf

[Dr. Harout]

Thanks guys, I think I got it now.