Vignetting is a brightness reduction when going from the photo center towards the periphery.
Usually considered a photographic defect induced by several elements within the optics or the sensor, vignetting is also used as an element of style (via photo-editing or removal) to mimic old-style photography that exhibited these flaws or to add an emphasis.
In the context of our camera hardware analysis and quality benchmarks, artificial vignetting filters don’t matter.
When gauging a camera’s image quality, excess vignetting is a negative aspect, as the data quality drops as you go farther from the image’s center.
To hide vignetting, the camera maker might use various filters to prop up the brightness artificially, without improving the details, dynamic range, or noise in vignetted these areas.
Vignetting is encountered most frequently with short focal lengths “Wide” (~24mm) and “Ultrawide” (~12mm) lenses as they naturally present a difficult challenge to gather and distribute light evenly all the way to the sensor’s corners.
As you can easily imagine, the light going straight into the center of the sensor creates a stronger sensor signal than light coming at an angle at the periphery. This is Pixel Vignetting and is the most common case for mobile cameras.
Pixel Vignetting can be exacerbated if a relatively large sensor (for specifications marketing) is paired with a lens with a tiny aperture (for cost-reduction).
Zoom camera modules are rarely affected by this, and some lenses have even been explicitly designed to reduce or eliminate vignetting. Ultrawide cameras with large sensors are often paired with a narrower field of view lens, making them “less ultrawide” (18mm vs. 13mm).
In large digital cameras, it is also possible that the aperture is so big that some of the light is blocked by the lens’s barrel. It is called optical vignetting, and we haven’t seen evidence of it on mobiles.
Opto-e has an in-depth explanation of it if you are curious. I also really like the paper from cmu.edu on the topic that goes into the details and high levelmath.
Some vignetting-looking effects can also come from having a lens hood that is too long or with certain combinations of lens filters.
That is called mechanical vignetting, but mobile cameras are typically not affected as they have virtually no such accessories.
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