Explain this, if you can !

[Wavytone]

Came across an interesting optical effect... Take a close look at the horse in photo no. 5 on the page http://www.theatlantic.com/infocus/2...1-of-3/100418/

Now, there is obviously a mesh screen between horse and camera, so why the fringes ?

Anyone able to volunteer a cogent optical explanation ? PS I know, but curious to see if other IIS members can figure it out.

[naskies (Dave)]

Looks like a Moiré pattern to me?

[Wavytone]

The usual moire effect is produced by two screens overlapping ad the fringes seen are caused by relative misalignments in the grid of each screen, usually more-or-less straight lines with some curves that aren't a contour map of a physical object.

So how does one single fabric screen produce fringes that are clearly a contour map of the horses anatomy, similar to what an interferometer would produce ?

[naskies (Dave)]

A horse is 3D... unlike a (mostly) 2D screen?

[okiscopey (Mike)]

Originally Posted by Wavytone

So how does one single fabric screen produce fringes that are clearly a contour map of the horses anatomy, similar to what an interferometer would produce ?

The single fabric screen is casting a shadow of itself on the horse. The shadow acts as another 'screen', and because this shadow screen is not on a flat surface, there are variable moire effects which act to show the horse's contours.

This was one technique used in medical and other technical photography work before laser scanners were invented.

[wasyoungonce (Brendan)]

It's quite possible the Anti-Aliasing filter has been removed from the DSLR sensor, deliberately to give this effect.

I did read about this ages ago with some Astro users taking this off as has no requirement for astro images.