Color highlights in stacked image

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mkeilly
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Location: Erie, Colorado, USA

Color highlights in stacked image

Post by mkeilly »

I am experimenting with a new focus rail (Neewer) and using a Canon EOS Rebel T5 with a 50mm Canon macro lens and diffuse light boxes. Just finished photographing a ring and ended up with some purple highlights. Anybody know why and how I can eliminate them?

Thanks,
Michael
Attachments
ring.small.jpg

Chris S.
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Re: Color highlights in stacked image

Post by Chris S. »

Michael, welcome to the forum! :D

That purple fringing in areas of high contrast looks like a classic presentation of longitudinal chromatic aberration (aka axial chromatic aberration). This LCA comes from your lens, and is difficult to correct in post production.

Some lenses that exhibit LCA lose some of it if you stop them down a bit. Of course, this will also increase diffraction, so you might lose a bit of resolution as a trade-off.

I don't know what aperture you used on your 50mm macro lens, but if it was wide open, you might try closing it down one or two stops. If it was already in the middle of the f-stop range, further closing down isn't likely to help much.

Many of us, here, have faced the dreaded purples, and don't like them at all. With high-contrast subjects, one sometimes has little choice but to upgrade the optics, moving to (unfortunately expensive) apochromatic (aka "apo") lenses. True apochromates should be very well corrected against LCA and another form of chromatic aberration called "transverse" or "lateral" CA. Beware, though, that many lenses marketed as "apo" are not truly apochromats.

With lower contrast subjects, LCA may not be as noticeable. Also, in portions of a stacked images where there is competing, in-focus detail, the stacking algorithm may choose non-aberrated portions of the stack because they are less fuzzy. Because of this, for some subjects, much LCA may disappear during stacking. For remaining regions of LCA, it may be possible to retouch them by hand. (This tends to be tedious.)

Your subject, that shiny ring, would be a difficult one to shoot with an optic that exhibits LCA. You might try lowering your exposure, which might give you a few more elements of detail to compete with the out-of-focus purple fuzz. This said, since the jewel in the ring should almost certainly have elements that are sparkly white, I doubt this would help all that much.

--Chris S.

rjlittlefield
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Re: Color highlights in stacked image

Post by rjlittlefield »

Michael, welcome aboard!

I see that Chris S. has been writing at the same time that I have.

I agree, the purple blotches look like longitudinal chromatic aberration (LoCA). It happens because the lens focuses some colors in different planes than other colors. Typically the most affected colors are at the extreme end(s) of the spectrum, colors that render as deep blue shading into purple. The blue-purple colors are often seen in regions that are dark and featureless, where even a small amount of unfocused light bleeding in from nearby bright regions can make a large relative change in the light that appears to come from those regions.

LoCA varies a lot from one lens to another, so if you have multiple lenses that can take the type of photo you want, you should see check to see if one does a better job than the others.

Different forms of illumination also can affect the severity of the problem. The illumination from some LEDs is especially bad, because of a strong peak in the blue part of the spectrum.

Once a stack has been shot with LoCA in it, you have a couple of options.

First, you should check the individual source images to see if any of them have a better rendition of the troublesome area than the stacked composite does. That's not common, but if it happens, then you can use retouching within the stacking software to use the better captures in those areas, rather than what the automatic stacking process used.

After that, your best option is to reduce the color saturation in the affected areas. In many scenes, like yours, the blue-purple color due to LoCA does not appear naturally anywhere else in the picture. In those cases, it can often be addressed automatically by a tool in your general purpose photo editor. An internet search along the lines of photoshop remove purple fringe will get you lots of information about that.

--Rik

mkeilly
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Re: Color highlights in stacked image

Post by mkeilly »

Thanks Chris and Ric. I appreciate the information, but I wish there was an easy fix!

rjlittlefield
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Re: Color highlights in stacked image

Post by rjlittlefield »

Sorry, there are no easy fixes for LoCA.

Probably the closest to what you want are the "remove purple fringe" tools.

--Rik

soldevilla
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Re: Color highlights in stacked image

Post by soldevilla »

There is an approach to a solution that is not perfect, but at least it is cheap. I usually stack the same stack in both methods that Zerene proposes. If you examine the two images, one usually bursts the highlights a lot and the other shows fringes of chromatic aberration. I superimpose the one without CA on a Photoshop layer and change it to COLOR mode.

mkeilly
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Re: Color highlights in stacked image

Post by mkeilly »

Thanks to all for the help. I tried using the same lens stopped down and a different Canon lens also stopped down and had very little of the purple fringe show up. Not ideal, but an acceptable result.

Pau
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Re: Color highlights in stacked image

Post by Pau »

If you're using Canon camera with Canon EF lens you can shot RAW and convert it with DPP, in its lens correction menu if you mark the box named "color diffusion*" or similar (just under Chromatic Aberration) it diminishes a lot purple fringes in some cases
Pau

Lou Jost
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Re: Color highlights in stacked image

Post by Lou Jost »

There is an approach to a solution that is not perfect, but at least it is cheap. I usually stack the same stack in both methods that Zerene proposes. If you examine the two images, one usually bursts the highlights a lot and the other shows fringes of chromatic aberration. I superimpose the one without CA on a Photoshop layer and change it to COLOR mode.
I just discovered the same thing a few weeks ago. It is a nice technique that deserves to be more widely known. I find that Luminosity blending mode works best, and the DMap should be on top. This not only removes some chromatic aberrations (which are usually stronger in DMap) but also removes most astigmatism (which often shows up in the PMax images). When the Luminosity mode is used, the lights and darks (but not the colors) of the DMap are applied to the PMax image. The problems of the two types of stacking algorithms cancel each other out! Another useful mode is Multiply.

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