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

• B
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.

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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 :)

sophiecentaur
Gold Member
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.

phinds
Gold Member
2019 Award
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.
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.

A.T.
Hope it all makes sense :)
Yes, diffraction effects around an obstacle are much smaller, than what we see there.

sophiecentaur
Gold Member
See the response by Imari Karonen (next to number 335) to the question How Does Light Bend Around My Fingertip:
http://physics.stackexchange.com/questions/111006/how-does-light-bend-around-my-finger-tip

I didn't read through all of it, but at first glance the two ray diagrams and the associated comments seem to explain the effect.
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)

A.T.
How did you do that?
"share" link at the bottom of the post

sophiecentaur