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I came across this effect the other day, and have spent the better part of the last week trying to figure out exactly what is happening.

When photographing two layers, with the foreground in front of the focus plane and the background behind the focus plane, the foreground distorts the background. In-focus subjects are not affected. There also appears to be a slight sharpening effect as well.

Three photos presented:

  1. The phenomenon described :

    f/5.6, hand about a foot from lens, focus around 3 feet, background building 50 feet away

  2. The background without the foreground:

    Exposure different due to auto exposure

  3. The same shot with a smaller aperture:

    f/16. Effect still visible but greatly reduced

Can anyone explain what this effect is? Does it have a name? Is it limited to certain situations beyond what I described? How can I learn more about this?

Since noticing this, I've started seeing it all over the place, including movies and even the naked eye.

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  • \$\begingroup\$ I'm sure womeone will come with a complete explanation. But mainly is due diffraction. \$\endgroup\$
    – Rafael
    Commented Apr 22, 2015 at 19:41
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    \$\begingroup\$ This isn't just an effect you can see by taking a picture with a camera. You can also see it with your naked eye. It is diffraction caused by the fingers in the light path. \$\endgroup\$
    – Michael C
    Commented Apr 22, 2015 at 23:52
  • \$\begingroup\$ I thought diffraction only happens at small apertures. I'd love to find more information about diffraction at the macro scale if that's indeed what this is \$\endgroup\$
    – mhlester
    Commented Apr 23, 2015 at 19:26

1 Answer 1

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No, this is not diffraction.

Let's start by recalling how the image is formed by the lens (focused and defocused): Focusing

Each point of your large aperture lens contributes to just one point of the defocused image: Single ray in a defocused image

(by the way, this also shows why aperture size affects (de)focus)

And what happens if you place an obstacle (your hands) near the lens? Not all defocused rays can reach our image, as a result the image appears more focused and slightly shifted. Defocused image + obstacle

And this is what happens in your photo - by covering the aperture, parts of the background become shifted and warped, depending on the obstacle shape.

...and now I found the same effect on physics.SE, having much better explanation and pictures!

https://physics.stackexchange.com/questions/111006/how-does-light-bend-around-my-finger-tip

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  • \$\begingroup\$ Very helpful, thank you! Do you know of a name for this effect? \$\endgroup\$
    – mhlester
    Commented Jun 22, 2015 at 16:10
  • \$\begingroup\$ I don't know... but now i found the same effect on physics SE - with much better explanation and pictures! \$\endgroup\$
    – szulat
    Commented Jun 22, 2015 at 16:35
  • \$\begingroup\$ And now the question is - can this question be marked as a duplicate with the proper link, or such things do not work across different sites? \$\endgroup\$
    – szulat
    Commented Jun 22, 2015 at 16:41
  • \$\begingroup\$ @mhlester Well, it's an incomplete aperture, or more an aperture "block" than a ring. \$\endgroup\$
    – sebix
    Commented Sep 5, 2015 at 18:04

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