Understanding CG Bokeh

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Bokeh is a photographic lens effect in which distant bright spots of light, blur into a variety of shapes based on the aperture in the lens. Bokeh can come in an infinite variety of shapes, softness, brightness and colours. And each type of bokeh gives a completely different effect to the image. 

When you look at a fairly lights in the distance and you blur out your eyesight by focusing on something close by — take a moment to analyze your out-of-focus area. 

Circles with a soft core and a gradient of rings glow agains the blurry background.
These are the blur disks of your eye. And that is the bokeh of your image. 

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Camera-lenses or your human eyes have varying degrees of optical aberrations.
This makes every image unique in the way the blur looks and more importantly, feels.

You can make an image feel magical through the use of bokeh. And you can also make it feel claustrophobic or disorienting.
The out-of-focus areas of your image take on a magnified shape of the light passing the lens elements. As the light ultimately passes through the aperture it has been filtered and cut into the shape of your bokeh. 

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In Computer Graphics that is no different. Bokeh is calculated during the 3D-rendering process based on the same properties.
F-Stop, aperture shape, vignetting, anisotropy and overlay textures define the lens characteristics.

These settings give you all the flexibility you need to create the textured out-of-focus areas in your render. 

Artists can use this for additional creative effects as well. CG bokeh is essence using a sprite to process the light rays hitting the camera.
The more a ray is out-of-focus, the more the bokeh sprite will be magnified in front of it. This means that one can add an animated image sequence as their bokeh map. 

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Imagine, an aperture map sequence that starts of with a soft-core and a ring of light around the edges. But after a certain number of frames the aperture map starts changing.
Introducing chromatic aberration or darkening in the core. 

This added flexibility can come in handy when in need of artistically timed effects. 

Pixar made use of this technique very effectively in Encanto (2021). There they used animated bokeh to emphasize the magical realism of fire.
Floating embers and sparks could transform as they became out of focus. Or they could have an animated glint effect as mentioned in this SideFX Houdini article. 

CG Bokeh gives animators and look-dev artists even more control in creating the scene you intend to make. Important aspects of cinema such as conveying emotions and storytelling, can be aided through the bokeh of a scene. And in CG you will find an added dimension to this creative process. 

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