Bokeh highlights turn into cat’s-eye shapes near the frame edges because the lens barrel and internal element mounts physically clip part of the light cone when it enters at an angle, a form of mechanical vignetting, while onion-ring texture inside the blur itself comes from a separate cause: fine concentric structure left on the surface of ground or molded aspherical elements during manufacturing. Neither effect is automatically a flaw; both are normal geometric and manufacturing side effects that vary in strength with lens design, distance from center, and aperture. Understanding the two separate mechanisms lets you judge whether a given lens’s out-of-focus rendering fits the images you want to make, rather than treating either shape as an automatic defect.
Create a Useful Highlight Test
A reliable way to evaluate both effects is to photograph small, distant, genuinely point-like light sources at night, such as string lights or streetlights spread across a range of distances from the center of the frame, while focusing well in front of them so they render as clearly defocused blur discs. Keep the same subject distance, focal length and aperture noted for each comparison frame, since changing any of these changes the size and character of the blur independent of the lens’s inherent rendering, which would make a side-by-side comparison meaningless.
Cat-Eye Shapes Near the Edges
Near the center of the frame, light from every part of the aperture opening reaches that point on the sensor without obstruction, so the defocused highlight renders as a full circle, or matches the shape of the aperture blades if stopped down. Toward the edges and corners, the story changes: the lens barrel, hood mount, or internal element rims physically block part of the oblique cone of light arriving from that angle, an effect called mechanical vignetting. Because the blockage happens unevenly around the aperture opening, the highlight takes on a lens-shaped or eye-shaped outline rather than a circle, commonly called cat’s-eye bokeh.
This effect is strongest wide open and toward the far corners, and it typically reduces or disappears as the lens is stopped down a stop or two, since a smaller working aperture opening sits farther from the barrel edges causing the clipping.
Rings Inside a Blur Disc
Onion-ring texture describes faint concentric rings visible inside an otherwise smooth-looking blur disc, and it comes from a different source than cat’s-eye clipping. It is most commonly associated with aspherical elements produced by grinding or molding, processes that can leave microscopic annular tool marks or diffraction-related structure on the aspherical surface itself; when light passes through that surface, the structure becomes visible as texture within out-of-focus highlights.
This is distinct from two other things that can also change the look of a highlight: the straight-edged polygon shape produced by stopped-down aperture blades, and colored fringing at the rim of a blur disc, which comes from chromatic aberration rather than surface texture. Identifying which of these three causes, mechanical clipping, surface texture, or blade shape and color fringing, is producing a given look helps avoid mislabeling one effect as another.
Compare at Matched Conditions
Judging whether one lens renders bokeh better than another only makes sense when subject distance, focal length and framing, and therefore magnification, are matched, and when the images are viewed at the same output size, since a longer focal length or a closer subject at an identical f-number produces a visibly larger, often smoother-looking blur purely from increased defocus, independent of anything about the lens’s rendering character. Cat’s-eye shapes and onion-ring texture are best treated as characteristics of a lens’s rendering style, useful to know in advance for choosing the right tool for a given subject, rather than as a single defect that every viewer or use case scores the same way.