Photos of and with the GR...

...and some comments (in German):

http://forum.digitalfotonetz.de/viewtopic.php?t=101006

Modern Art

Yeah, I guess you know why this happens with this camera...
There are ways around it though. Obviously you can correct it using the in-camera moire correction but you can also stop it occurring by using an aperture of f10 or smaller.

Yes, because of the lack of a Bayer-AA filter.
There have been reports that it is quite unlikely that one runs into problems.
Therefore I tried to provoke color moiré.
https://www.pentaxuser.com/forum/topic/k5-2s-moire-just-for-the-record-43272/p-3...

Not always and not with this pattern.
The in-camera moire correction fails: http://forum.digitalfotonetz.de/viewtopic.php?p=1313593#1313593
Try to correct it by yourself (note that this is an irregular repetition of a single small rectangle).
I am quite convinced that this is not so easy.
The original looks everywhere like the bottom of the left printout.
I shortly discussed there, why moiré occurs only "partially" (in German):
http://forum.digitalfotonetz.de/viewtopic.php?p=1313976#1313976
Btw, artefacts are visible also at f11.
However, you are right that smaller and larger apertures make it less apparent.
Changing subject distance also has an influence.
My point here is that the lack of a Bayer-AA filter can sometimes completely ruin your picture...

...or create Modern Art .

To be fair, it is a very extreme example in choice of subject.

With this image I wonder if you would not see some moire even with an AA filter?

To correct it though I would think about converting to black and white, or even shooting in black and white. Have you tried this approach, and if so does it help?

Yes, as I said, I tried to provoke the effect.
But it was easier than I thought.

I did not try this, but surely moiré would not be as prominent as here.
As I discussed shortly in one of the links (in German), you cannot prominently see color moiré everywhere in the picture.
This is due to the fact that the wall and the camera were not fully parallel.
Furthermore, each print was only fixed by one adhesive tape.
So, not everything is in perfect focus.
This tiny bit of "blurring" (which normally is the purpose of the Bayer-AA filter) is enough to make the pattern (nearly) disappear (see bottom part of left print-out and left part of right print-out).

This is also what the in-camera moiré correction seems to do partly: Desaturate problematic parts of the picture.
Convert the above picture into grayscale or black and white, you will see that this is not sufficient.
Shooting in black and white is not really possible, as the original color (Bayer) output will only be converted.
So, these two approaches are not so different after all.
Thus, more photoshopping would be required.

So, here we go (similar pattern & conditions):


K-3 + FA 20-35 (ISO 100, 20mm, f7.1, distance about 1.2m, AA-Filter-Simulator: OFF, TYPE 1, TYPE 2)
http://www.pentaxforums.com/forums/172-pentax-k-3/248432-k-3-aa-filter-simulator...

Would be interested in how well Paul's suggestion of using apertures smaller than f10 works to reduce moire (relying on diffraction to create blurring, rather than the SR-based AA-filter-simulator). Cos it sounds like a rather clever idea to me.

What f-stop do you need to go to to reduce the moire acceptably?

Moiré is less pronounced at small and at big aperture settings.
One could use MTF diagrams as a guideline to guess the corresponding f-stops for a specific lens.
At open aperture picture quality might not be sufficient.
Diffraction also can act as pseudo-AA-filter.
However, image quality then also is affected.
Other ways to reduce moiré: faint defocus (see above), a slightly different shooting position,...