Among the many features that make anamorphic lenses unique and beloved are the shape of the bokeh balls. Those out-of-focus highlights that stretch to be a circle on spherical lenses, instead stretch to be an oval on anamorphic lenses.
But have you ever taken the time to understand why? Of course, anamorphics squeeze the image, but objects in focus don't turn into ovals, so why do objects out of focus?
Filmmaker IQ is here with the answer, and while it involves math, they use a lot of diagrams to make it easier to understand.
The simplest version of the answer is that "out-of-focus areas expand more than in-focus areas in direct proportion to the lens being used for the shot."
A longer lens is going to create more "spread" than a wider lens for the bokeh balls. An anamorphic system can be thought of as having two lenses, one for the horizontal field of view, one for the vertical. Thus the "spread" of the bokeh balls will be different for each axis, with there being more spread vertically (when behind the focal plane, as they typically are) than horizontally. This makes ovals.
One beauty of this video is also its explanation of why this only works if the anamorphic element is in front of the iris ring, as it is in most prime lenses.
In some lenses (often zoom lenses), the anamorphic element is behind the iris, which doesn't create this effect. This has led to the misunderstanding that it's the iris itself which creates the ovals, but that isn't true.
But if you've ever wondered exactly why rear element anamorphics don't feel as "anamorphic" front element, now you've got a deep dive to help you.
If you are an anamorphic shooter, this video is absolutely worth your time. It might not affect your everyday decision on set, but always good to have a better understanding of how the images you craft end up looking a certain way. It also has a nice explanation of how anamorphic lenses create those streak flares (astigmatism, same as your optometrist might diagnose). And it's always good to have an explanation if a client or collaborator asks about why an image looks a certain way.
Have you ever shot anamorphic? Tell us about the experience!
Source: Filmmaker IQ