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I took some photos with my a7 iii and a Tamron 28-75 lens yesterday and one of the subjects was wearing a shirt made with pink and white fibers. I'm seeing some weird artifacts in the colors that I've not seen before. I do have a UV filter on the lens which is the only thing I can think of. Any ideas what this is?

Also any ideas on how I might be able to fix this in Photoshop? I have no idea where to start.

pink shirt with artifacts

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    \$\begingroup\$ Does this answer your question? What is a low pass filter, and does a camera need to have one? \$\endgroup\$
    – Michael C
    Dec 16, 2022 at 3:59
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    \$\begingroup\$ The Sony α7 III has a "half" low pass/AA filter. It has an horizontal AA filter, but not a vertical AA filter. Traditionally, cameras with low pass/anti-aliasing filters have both a horizontally oriented and a vertically oriented filter oriented 90° to one another. Many cameras with "no" AA filter actually have two oriented at 180° to each other. The second cancels the effect of the first. Manufacturers did this to simplify production of the same basic model with one variant having two AA filters at 90° to one another, the other variant with the filters at 180° (e.g. Nikon D800/D800E). \$\endgroup\$
    – Michael C
    Dec 16, 2022 at 4:08
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    \$\begingroup\$ Do you see the moiré pattern when zoomed in 100%? Scaling algorithms can introduce their own moiré... \$\endgroup\$
    – xenoid
    Dec 16, 2022 at 8:51

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It's color moire... notice that it's just blue and reds but not greens, which means it is occuring at 1/2 pixel frequency/spacing.

To eliminate it you need to change the pattern of the fabric so that it is significantly different than the pattern of the red/blue pixels on the sensor... this can be done in several ways.

Blurring is a primary method... stopping down into diffraction, an intensional slight defocus, or a soft focus filter. I'm not aware of any add-on AA filter options.

Another is to change the size of the pattern by changing it's magnification... change the FL/Zoom/distance.

I've often read that fixing moire in post is easy; but IME it is seldom terribly effective... particularly with color moire.

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It't an interference pattern, also called moire pattern.

The cause is, that the pattern on the shirt has almost the same size as the pixels on your sensor. The small deviations lead to an interference pattern that looks like what you see.

You can get rid of those patterns with a low pass filter in front of your sensor. Getting rid of those patterns in post processing is very hard.

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