Anamorphic bokeh isn't oval because someone decided ovals look more cinematic. It's oval because a cylindrical piece of glass doesn't know the difference between the light forming your subject and the light forming the blur behind it — it compresses both, because it compresses everything that passes through it.
What the Squeeze Actually Does
A spherical lens curves the same way in every direction around its axis — light entering from any angle around the lens gets bent by the same amount for that angle. An anamorphic lens breaks that symmetry on purpose, using one or more cylindrical elements that curve in one axis and stay flat in the other. The result is that the lens compresses the image horizontally by some fixed squeeze factor — commonly 2x, sometimes 1.3x or other ratios depending on the design — while leaving the vertical dimension essentially untouched. That compression is what let filmmakers capture a wide field of view onto film stock that wasn't physically any wider than what a spherical lens used, with the image squeezed to fit and then stretched back out to its intended width at projection or in post.
The important part for what happens next: that compression isn't applied to "the image" as some already-finished thing. It's applied to every ray of light passing through the lens, at the moment it passes through, regardless of what that ray is doing — converging sharply toward focus, or spreading out because it's nowhere near the focal plane.
Why the Blur Circle Doesn't Stay a Circle
In a spherical lens, an out-of-focus point of light spreads into a blur circle because the cone of light converging toward (or diverging from) the focal point is itself circular in cross-section, and the lens treats that circle the same in every direction. Nothing about the optics singles out horizontal from vertical.
An anamorphic lens's cylindrical elements don't stop compressing once a ray is out of focus — they have no way to know that. The same horizontal compression that squeezes the in-focus image also squeezes the cross-section of an out-of-focus light cone, in exactly the same axis and by exactly the same factor. A blur circle that would have been round through a spherical lens gets compressed along its horizontal dimension and left alone vertically, and what comes out the other side is an oval, stretched along the axis the compression didn't touch relative to the axis it did. This isn't a side effect the lens designers tolerated. It's the same mechanism responsible for the squeeze doing exactly what it always does, applied to light that happens not to be in focus.
The Streak Flares Come From the Same Elements
Anamorphic flares — the long horizontal blue streaks that shoot across the frame from a bright light source — trace back to the same cylindrical elements, though through a different part of their behavior. Internal reflections and light scattering off the cylindrical surfaces don't scatter uniformly in every direction the way a spherical surface's reflections would; the cylindrical geometry favors scattering along the axis it's shaped around, stretching stray light into a streak running horizontally across the frame rather than spreading into a symmetric halo. The characteristic blue-green cast in these streaks comes from the specific coatings and glass used in anamorphic designs, a separate detail from the streak's shape, which is set by the cylindrical geometry itself.
Both signatures — the oval highlight and the horizontal streak — come from the same design choice: glass that treats one axis differently from the other, in every interaction it has with light, not only the interaction that ends in a sharp image. For how the entirely different mechanism of blade-shaped bokeh works in a standard spherical lens, see our piece on aperture-shaped bokeh.
Nothing about an anamorphic lens is round by nature. What looks oval or streaked isn't a rendering choice happening after the image forms — it's the geometry of the glass showing up in whatever light passes through it, focused or not. You can compare how different optical signatures carry into a grade in the Cineon studio.