# B What´s the optical effect happening in this picture?

1. Aug 3, 2016

### lennon

Hi, I was watching a tv show and I found an optical phenomenon that puzzled me.
I´d say it´s related with the bokeh produced in the out of focus range, but I still can´t figure it out. I guess what makes it so weird is that the effect happens in relation with an object in the foreground (infront of the glass, instead of behind it). Since it´s obviously an effect of that glass (as caputred with that lens), I´d say that there´s something particular in the way in wich the rays are beign deflected after they go trhough it´s rectangular cells. Still it´s almost as if they´d move in a curved trayectory.
Well, anyway, I was hoping some of you may have a proper explanation, it surley is an interesting physics puzzle.

2. Aug 3, 2016

3. Aug 3, 2016

### Delta Kilo

While it does look a bit like a diffraction, I don't think it is. I think it all can be explained from thin lens perspective as a combination of 2 bokeh effects.
There is an unidentifiable object in the foreground and a window in the background, both out of focus, like top and bottom cases on the diagram:

As a result, there is a kind of alpha-blending happening between foreground and background. (This needs a better diagram) What you get is a sum of linear ramp from the foreground object and a periodic ripple from the venetian blinds (this needs another diagram). Plus when the sharp edge of the foreground object blocks some of the rays from the background, it acts as a kind of aperture for the background, effectively reducing background blur in vertical direction so the blinds in this area become sharper (this really needs a proper ray diagram :) On top of it, the window is grossly overexposed, but when blended with dark foreground color, it gets into valid intensity range and you start seeing finer details.
Hope it all makes sense :)

4. Aug 3, 2016

### sophiecentaur

The gaps and the angles make it unlikely to be diffraction. A convex lens can produce a spatial fourier transform of an object. For instance, a defocussed star will produce an Airey disc image in the eyepiece, for instance. The blind consists of a grille pattern with a certain spatial frequency and, when it passes through the camera lens, the resulting pattern can be a combination of the spatial variation and the spatial frequency. I have a feeling that the non linearity of the sensor could be making a contribution (?)
DeltaKilo is suggesting a similar thing (I think) but using different words. In any case, I think it's an artefact of the lens and the blind would look fine if the camera were focussed correctly on it.

5. Aug 3, 2016

### phinds

Exactly what I thought immediately. I didn't even understand what the puzzle was. Maybe because I've experimented some with camera depth of focus.

6. Aug 4, 2016

### A.T.

Yes, diffraction effects around an obstacle are much smaller, than what we see there.

7. Aug 4, 2016

8. Aug 4, 2016

9. Aug 4, 2016

### pixel

10. Aug 4, 2016

### sophiecentaur

The diagrams in that link show how a phase shift in the spatial distribution (the image) is caused when part of the spatial fourier transform is subtracted. This is the equivalent to what would happen if you took a square wave signal and blanked some of it; the peak in the output signal would be delayed (equivalent to shifting).
This link shows the fourier transform that a lens achieves. (Go to the last slide for a summary. lol)

11. Aug 4, 2016

### A.T.

"share" link at the bottom of the post

12. Aug 5, 2016

### sophiecentaur

Full marks for the guy who produced that web page with diagrams and actual pictures, taken with his DSLR. He certainly put his money where his mouth is. The explanation applies to so many optical phenomena which I had always assumed were down to diffraction*. It seems that they're all down to what our own eye optics do with an image where there a path that's partly occluded. At the risk of boring people with more about my recent purchase of an astronomical telescope. I now can explain that nasty aberration you get from a twig on a nearby tree that gets between the Moon and the telescope. You can't see the twig but the view of the Moon is annoyingly distorted. And that's when the twig is probably less than 10mm and the aperture is 200mm!
* Of course, in the wider scheme of things, every optical process can be described in terms of Diffraction - so I guess I really mean 'diffraction fringes'.

13. Aug 6, 2016

### mister mishka

Very interesting to read how this effect actually works in detail, I actually use this out of focus technique a lot for my art films. You can really play around with the "space" of the film, having several scenes at once where you can just shift focus to. I will definitely read more into this and think up some new interesting ways to film and experiment with this, but here is one of my films to show you how I use this effect:

https://vimeo.com/mistermishka/space001