Why does an out-of-focus foreground object seem to warp or sharpen the background?
Asked 4/22/2015
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When I place a nearby object in front of the lens, but keep focus on a subject farther away, the blurred foreground sometimes makes the background look locally warped or slightly sharper. With a wider aperture the effect is stronger; stopping down reduces it a lot. What causes this, does it have a name, and why does it only seem to affect out-of-focus areas?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
6
No, this is not diffraction.
Let's start by recalling how the image is formed by the lens (focused and defocused):
Each point of your large aperture lens contributes to just one point of the 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.
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
Originally by user32811. Source · Licensed CC BY-SA 4.0
user32811
11y ago
0
Generated from our catalog & community — verify before relying on it.
What you’re seeing is not diffraction. It’s an aperture-occlusion effect caused by a nearby object blocking part of the lens entrance pupil.
For out-of-focus background detail, each scene point is rendered as a blur circle formed by rays passing through the whole aperture. If a foreground object sits very close to the lens, it blocks some of those rays. That effectively cuts off part of the blur circle, which can make the background behind it appear shifted, warped, or even a bit “sharper” because the blur is reduced asymmetrically.
In-focus subjects aren’t affected the same way because their image points are formed much more precisely; the partial blockage mainly changes brightness rather than creating obvious local distortion.
The effect is strongest when:
- the foreground object is very close to the lens,
- the background is out of focus,
- the aperture is wide.
Stopping down reduces the effect because the aperture is smaller, so there’s less blur-circle area to clip.
It doesn’t require any special lens; it’s a geometric optics effect related to partial obstruction of the pupil by an out-of-focus foreground object.
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AI11y ago
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