Understanding Lens Aberrations in Optics

The beauty of the imperfect

Reading time
5 min
Published on

February 10, 2024

Blauw Films

Today we're going to talk about bokeh. You've most definitely seen these out of focus shapes of light.
Bokeh describes the feel of your blur. And an important distinction to make, is that it does not relate to how far something is out-of-focus.

Differences in Lens Aberrations and aperture shape cause very different bokeh effects.
As well as your decision of shooting with spherical or anamorphic lenses.

Let's first actually look at the difference between a spherical and an anamorphic lens by analysing what they do to the magnification of light.
To make an anamorphic lens in 3D I'm using a Cylinder from which I'm cutting a straight cube. This will act as our anamorphic lens element.

For a Spherical Lens we are doing the same with a sphere: 

I've given them a simple refractive material with Caustics enabled.
That will allow us to shine light through the element and se how it is bent by the lens.

As you can see, the Cylindrical Lens element stretches the light

Anamorphic lenses make use of cylindrical lens elements and spherical lenses use spherical elements.
On an abstract level that is what creates the distinct look between them.

Now let's talk about aberrations in lenses.
Simply put, aberrations are imperfections in your lens that add texture and details to your image.

There are two important categories of aberrations: 

  1. Monochromatic aberrations
  2. Chromatic aberrations

Monochromatic aberrations include spherical aberration, coma, astigmatism, and field curvature and image distortion.
These are all the optical effects that change the perfection of the light-rays hitting the sensor.

Chromatic aberrations are the more obvious ones to our eye as they shift the wavelengths of coloured light.
Thus creating a mis-alignment of the different colours on the sensor.

Spherical Aberrations are primarily responsible for the distribution of light within a lens.
This means that even though all the light coming through the lens meets nicely at a tiny point on the sensor or film, the light distribution within the cone itself may be uneven. This aberration causes the blur-circles to be unevenly lit, and instead they tend to collect more light in the middle of the discs or towards the edges.

Here we can see these effects come into action with another test scene:

In the example above we are working with

  1. a hexagonal aperture map
  2. with over-compensated light distribution
  3. a slight bit of chromatic aberration
  4. and strong monochromatic aberration (causing a cat-eye bokeh) 

All of these details should be considered when working on your 3D-renders.

Bokeh is the feel of your blur.

And thus it has a fundamental impact on how your shot is perceived by you and the viewer.
The decision of how you want it to look and feel is fully yours.

To accelerate our own process of making realistic bokeh we made the Bokeh Builder

Be sure to check it out on the Store and start creating beautiful depth of field!

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